diff --git a/src/App.css b/src/App.css
index 89d5f94..93484f9 100644
--- a/src/App.css
+++ b/src/App.css
@@ -54,6 +54,13 @@
   min-height: 52px;
 }
 
+.timer-footer-controls {
+  display: flex;
+  gap: 8px;
+  align-items: center;
+  flex-shrink: 0;
+}
+
 /* Small settings gear */
 .timer-settings-btn {
   width: 28px;
@@ -84,6 +91,41 @@
   cursor: not-allowed;
 }
 
+.timer-mic-btn {
+  width: 28px;
+  height: 28px;
+  padding: 0;
+  border: none;
+  border-radius: 50%;
+  background: transparent;
+  color: var(--color-text-faint);
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  transition:
+    color 0.15s,
+    background 0.15s;
+}
+
+.timer-mic-btn:hover:not(:disabled) {
+  color: var(--color-btn-text);
+  background: var(--color-settings-hover-bg);
+}
+
+.timer-mic-btn.active {
+  color: #53b36c;
+}
+
+.timer-mic-btn.detected {
+  color: #e8a838;
+}
+
+.timer-mic-btn:disabled {
+  opacity: 0.3;
+  cursor: not-allowed;
+}
+
 /* GitHub / sponsor links */
 .timer-display-links {
   display: flex;
diff --git a/src/App.tsx b/src/App.tsx
index 06c83ef..05b877e 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -1,6 +1,7 @@
 import { useRef, useState, useCallback, useEffect, useMemo } from 'react';
 import { useAppStore, useSettingsStore } from './store';
 import { usePhaseRunner } from './hooks/usePhaseRunner';
+import { useAudio } from './hooks/useAudio';
 import { useWakeLock } from './hooks/useWakeLock';
 import { useTheme } from './hooks/useTheme';
 import { useUrlParams } from './hooks/useUrlParams';
@@ -11,6 +12,7 @@ import { Gen4Panel } from './components/Gen4Panel';
 import { Gen3Panel } from './components/Gen3Panel';
 import { CustomPanel } from './components/CustomPanel';
 import { SettingsDialog } from './components/SettingsDialog';
+import { resumeAudioSync } from './audio/sounds';
 import './App.css';
 
 const TAB_LABELS = ['Gen 5', 'Gen 4', 'Gen 3', 'Custom'];
@@ -27,7 +29,16 @@ export default function App() {
   const running = useAppStore((s) => s.running);
   const setPhases = useAppStore((s) => s.setPhases);
 
-  const { toggle, registerFlash } = usePhaseRunner();
+  const { start, toggle, registerFlash } = usePhaseRunner();
+  const { isListening, isDetected, startListening, stopListening } = useAudio({
+    onDetect: () => {
+      if (useAppStore.getState().running) return;
+      start();
+      stopListening();
+      setStatusMessage('Sound detected — timer started.');
+      setTimeout(() => setStatusMessage('Ready'), 4000);
+    },
+  });
   useWakeLock();
   useUrlParams();
   useTheme();
@@ -79,7 +90,7 @@ export default function App() {
       setStatusMessage('Calibration applied.');
       setTimeout(() => setStatusMessage('Ready'), 4000);
     }
-  }, [running, currentRef, updatePhases]);
+  }, [running, currentRef, updatePhases, setStatusMessage]);
 
   const handleReset = useCallback(() => {
     if (running) return;
@@ -90,15 +101,33 @@ export default function App() {
   }, [running, currentRef, updatePhases]);
 
   const handleToggle = useCallback(() => {
+    if (!running && isListening) {
+      stopListening();
+    }
     toggle();
-  }, [toggle]);
+  }, [running, isListening, stopListening, toggle]);
+
+  const handleAudioToggle = useCallback(() => {
+    if (running) return;
+    if (isListening) {
+      stopListening();
+      setStatusMessage('Audio trigger stopped.');
+    } else {
+      resumeAudioSync();
+      startListening().then(
+        () => setStatusMessage('Listening for sound to start timer...'),
+        (err: unknown) =>
+          setStatusMessage(`Microphone error: ${err instanceof Error ? err.message : err}`),
+      );
+    }
+  }, [running, isListening, startListening, stopListening, setStatusMessage]);
 
   const handleSettingsClose = useCallback(
     (accepted: boolean) => {
       setSettingsOpen(false);
       if (accepted) setTimeout(updatePhases, 0);
     },
-    [updatePhases],
+    [updatePhases, setSettingsOpen],
   );
 
   // Keyboard shortcuts
@@ -133,6 +162,13 @@ export default function App() {
     prevRunning.current = running;
   }, [running]);
 
+  // Stop microphone when a run becomes active.
+  useEffect(() => {
+    if (running && isListening) {
+      stopListening();
+    }
+  }, [running, isListening, stopListening]);
+
   return (
     <div className="app">
       <div className="app-container">
@@ -141,7 +177,10 @@ export default function App() {
           registerFlash={registerFlash}
           onToggle={handleToggle}
           onSettings={() => setSettingsOpen(true)}
+          onToggleAudioListening={handleAudioToggle}
           settingsDisabled={running}
+          audioListening={isListening}
+          audioDetected={isDetected}
         />
 
         {/* Tab panel */}
diff --git a/src/audio/sounds.ts b/src/audio/sounds.ts
index d19a266..b452495 100644
--- a/src/audio/sounds.ts
+++ b/src/audio/sounds.ts
@@ -145,6 +145,11 @@ function ensureRunning(): Promise<void> {
   return resumePromise;
 }
 
+/** Fire-and-forget wrapper for use in event handlers. */
+export function resumeAudioSync(): void {
+  resumeAudio();
+}
+
 // ─── Playback (fire-and-forget) ───
 
 function playBuffer(buffer: Promise<AudioBuffer>, label: string, receivedAt: number): void {
diff --git a/src/components/TimerDisplay.tsx b/src/components/TimerDisplay.tsx
index 33a777e..cc3000b 100644
--- a/src/components/TimerDisplay.tsx
+++ b/src/components/TimerDisplay.tsx
@@ -23,14 +23,20 @@ interface TimerDisplayProps {
   registerFlash: (fn: () => void) => void;
   onToggle: () => void;
   onSettings: () => void;
+  onToggleAudioListening: () => void;
   settingsDisabled: boolean;
+  audioListening: boolean;
+  audioDetected: boolean;
 }
 
 export function TimerDisplay({
   registerFlash,
   onToggle,
   onSettings,
+  onToggleAudioListening,
   settingsDisabled,
+  audioListening,
+  audioDetected,
 }: TimerDisplayProps) {
   const phases = useAppStore((s) => s.phases);
   const currentPhaseIndex = useAppStore((s) => s.currentPhaseIndex);
@@ -127,27 +133,54 @@ export function TimerDisplay({
         )}
       </div>
       <div className="timer-display-footer">
-        <button
-          className="timer-settings-btn"
-          onClick={onSettings}
-          disabled={settingsDisabled}
-          title="Open Settings (Ctrl+,)"
-        >
-          <svg
-            viewBox="0 0 24 24"
-            width="16"
-            height="16"
-            fill="none"
-            stroke="currentColor"
-            strokeWidth="2"
-            strokeLinecap="round"
-            strokeLinejoin="round"
-            style={{ display: 'block' }}
+        <div className="timer-footer-controls">
+          <button
+            className="timer-settings-btn"
+            onClick={onSettings}
+            disabled={settingsDisabled}
+            title="Open Settings (Ctrl+,)"
           >
-            <circle cx="12" cy="12" r="3" />
-            <path d="M19.4 15a1.65 1.65 0 0 0 .33 1.82l.06.06a2 2 0 0 1-2.83 2.83l-.06-.06a1.65 1.65 0 0 0-1.82-.33 1.65 1.65 0 0 0-1 1.51V21a2 2 0 0 1-4 0v-.09A1.65 1.65 0 0 0 9 19.4a1.65 1.65 0 0 0-1.82.33l-.06.06a2 2 0 0 1-2.83-2.83l.06-.06A1.65 1.65 0 0 0 4.68 15a1.65 1.65 0 0 0-1.51-1H3a2 2 0 0 1 0-4h.09A1.65 1.65 0 0 0 4.6 9a1.65 1.65 0 0 0-.33-1.82l-.06-.06a2 2 0 0 1 2.83-2.83l.06.06A1.65 1.65 0 0 0 9 4.68a1.65 1.65 0 0 0 1-1.51V3a2 2 0 0 1 4 0v.09a1.65 1.65 0 0 0 1 1.51 1.65 1.65 0 0 0 1.82-.33l.06-.06a2 2 0 0 1 2.83 2.83l-.06.06A1.65 1.65 0 0 0 19.4 9a1.65 1.65 0 0 0 1.51 1H21a2 2 0 0 1 0 4h-.09a1.65 1.65 0 0 0-1.51 1z" />
-          </svg>
-        </button>
+            <svg
+              viewBox="0 0 24 24"
+              width="16"
+              height="16"
+              fill="none"
+              stroke="currentColor"
+              strokeWidth="2"
+              strokeLinecap="round"
+              strokeLinejoin="round"
+              style={{ display: 'block' }}
+            >
+              <circle cx="12" cy="12" r="3" />
+              <path d="M19.4 15a1.65 1.65 0 0 0 .33 1.82l.06.06a2 2 0 0 1-2.83 2.83l-.06-.06a1.65 1.65 0 0 0-1.82-.33 1.65 1.65 0 0 0-1 1.51V21a2 2 0 0 1-4 0v-.09A1.65 1.65 0 0 0 9 19.4a1.65 1.65 0 0 0-1.82.33l-.06.06a2 2 0 0 1-2.83-2.83l.06-.06A1.65 1.65 0 0 0 4.68 15a1.65 1.65 0 0 0-1.51-1H3a2 2 0 0 1 0-4h.09A1.65 1.65 0 0 0 4.6 9a1.65 1.65 0 0 0-.33-1.82l-.06-.06a2 2 0 0 1 2.83-2.83l.06.06A1.65 1.65 0 0 0 9 4.68a1.65 1.65 0 0 0 1-1.51V3a2 2 0 0 1 4 0v.09a1.65 1.65 0 0 0 1 1.51 1.65 1.65 0 0 0 1.82-.33l.06-.06a2 2 0 0 1 2.83 2.83l-.06.06A1.65 1.65 0 0 0 19.4 9a1.65 1.65 0 0 0 1.51 1H21a2 2 0 0 1 0 4h-.09a1.65 1.65 0 0 0-1.51 1z" />
+            </svg>
+          </button>
+          <button
+            className={`timer-mic-btn${audioListening ? ' active' : ''}${audioDetected ? ' detected' : ''}`}
+            onClick={onToggleAudioListening}
+            disabled={settingsDisabled}
+            title={audioListening ? 'Stop audio trigger' : 'Start audio trigger'}
+            aria-label={audioListening ? 'Stop audio trigger' : 'Start audio trigger'}
+            aria-pressed={audioListening}
+          >
+            <svg
+              viewBox="0 0 24 24"
+              width="16"
+              height="16"
+              fill="none"
+              stroke="currentColor"
+              strokeWidth="2"
+              strokeLinecap="round"
+              strokeLinejoin="round"
+              style={{ display: 'block' }}
+            >
+              <path d="M12 1a3 3 0 0 1 3 3v8a3 3 0 0 1-6 0V4a3 3 0 0 1 3-3z" />
+              <path d="M19 10v2a7 7 0 0 1-14 0v-2" />
+              <line x1="12" y1="19" x2="12" y2="23" />
+              <line x1="8" y1="23" x2="16" y2="23" />
+            </svg>
+          </button>
+        </div>
         <button
           className="timer-play-stop"
           onClick={onToggle}
diff --git a/src/hooks/useAudio.ts b/src/hooks/useAudio.ts
new file mode 100644
index 0000000..438b9bd
--- /dev/null
+++ b/src/hooks/useAudio.ts
@@ -0,0 +1,120 @@
+import { useCallback, useEffect, useRef, useState } from 'react';
+
+const THRESHOLD = 60;
+const SILENCE_CENTER = 128;
+
+interface UseAudioOptions {
+  onDetect?: () => void;
+  threshold?: number;
+}
+
+export const useAudio = (options?: UseAudioOptions) => {
+  const [isDetected, setIsDetected] = useState(false);
+  const [isListening, setIsListening] = useState(false);
+
+  const audioContextRef = useRef<AudioContext | null>(null);
+  const analyserRef = useRef<AnalyserNode | null>(null);
+  const sourceRef = useRef<MediaStreamAudioSourceNode | null>(null);
+  const streamRef = useRef<MediaStream | null>(null);
+  const requestRef = useRef<number | null>(null);
+  const dataArrayRef = useRef<Uint8Array | null>(null);
+  const detectedRef = useRef(false);
+  const startingRef = useRef(false);
+  const onDetectRef = useRef<UseAudioOptions['onDetect']>(options?.onDetect);
+
+  useEffect(() => {
+    onDetectRef.current = options?.onDetect;
+  }, [options?.onDetect]);
+
+  function calculateMaxDeviation(dataArray: Uint8Array) {
+    let maxDeviation = 0;
+    for (let i = 0; i < dataArray.length; i++) {
+      const deviation = Math.abs(dataArray[i] - SILENCE_CENTER);
+      if (deviation > maxDeviation) {
+        maxDeviation = deviation;
+      }
+    }
+    return maxDeviation;
+  }
+
+  const analyzeSound = useCallback(() => {
+    const analyser = analyserRef.current;
+    if (!analyser) return;
+
+    const dataArray = dataArrayRef.current!;
+    analyser.getByteTimeDomainData(dataArray);
+
+    const maxDeviation = calculateMaxDeviation(dataArray);
+    const threshold = options?.threshold ?? THRESHOLD;
+    if (maxDeviation > threshold && !detectedRef.current) {
+      detectedRef.current = true;
+      setIsDetected(true);
+      onDetectRef.current?.();
+      return;
+    }
+
+    if (!analyserRef.current) return;
+    requestRef.current = requestAnimationFrame(analyzeSound);
+  }, [options?.threshold]);
+
+  const stopListening = useCallback(() => {
+    if (requestRef.current !== null) {
+      cancelAnimationFrame(requestRef.current);
+      requestRef.current = null;
+    }
+
+    sourceRef.current?.disconnect();
+    sourceRef.current = null;
+
+    analyserRef.current?.disconnect();
+    analyserRef.current = null;
+
+    streamRef.current?.getTracks().forEach((track) => track.stop());
+    streamRef.current = null;
+
+    if (audioContextRef.current) {
+      void audioContextRef.current.close();
+      audioContextRef.current = null;
+    }
+
+    dataArrayRef.current = null;
+    detectedRef.current = false;
+    setIsListening(false);
+  }, []);
+
+  const startListening = async () => {
+    if (isListening || startingRef.current) return;
+
+    if (!navigator.mediaDevices?.getUserMedia) {
+      throw new Error('Microphone access requires a secure context (HTTPS or localhost).');
+    }
+
+    startingRef.current = true;
+    try {
+      const stream = await navigator.mediaDevices.getUserMedia({ audio: true });
+
+      const audioContext = new window.AudioContext();
+      const analyser = audioContext.createAnalyser();
+      const source = audioContext.createMediaStreamSource(stream);
+
+      source.connect(analyser);
+
+      audioContextRef.current = audioContext;
+      analyserRef.current = analyser;
+      sourceRef.current = source;
+      streamRef.current = stream;
+      dataArrayRef.current = new Uint8Array(analyser.fftSize);
+
+      detectedRef.current = false;
+      setIsListening(true);
+      setIsDetected(false);
+      analyzeSound();
+    } finally {
+      startingRef.current = false;
+    }
+  };
+
+  useEffect(() => stopListening, [stopListening]);
+
+  return { isListening, isDetected, startListening, stopListening };
+};