[jfpaardoo]

What this changes

• Adds a configurable Audio output option under Options > Audio (in both the standalone Launcher and in-game menu), settings.ini, and the control socket (audio_output = stereo | surround_51 | headphones_3d):
  • Stereo (default): standard ITU 5.1-to-2.0 downmix (audio_output = stereo / 0), keeping default behavior untouched.
  • Surround 5.1: native 6-channel discrete surround sound (FL, FR, FC, LFE, BL, BR) passed directly to SDL3 (audio_output = surround_51 / 1).
  • 3D Headphones: binaural virtual-speaker spatializer (audio_output = headphones_3d / 2) implementing interaural time difference (ITD) delay lines, head-shadow low-pass filtering, rear pinna high-shelf filtering, center-channel early reflection, LFE ~120 Hz low-pass filtering, and a soft-knee limiter.
• Updates IAudioDevice::GetActiveChannelCount (cmd 5) and GetActiveChannelCountAuto (cmd 13) in runtime/services/audio.c to report 6 active channels when Surround 5.1 or 3D Headphones is selected, so the game's nn::audio engine renders discrete 6-channel audio.
• Signals IAudioDevice system events (cmd 4, 11, 12) on runtime mode changes and dynamically re-opens the SDL audio output stream when switching between 2-channel and 6-channel modes.
• Includes menu translations across all 13 runtime/ui/i18n/*.inc files and synthetic unit tests in tests/test_audio_mix.c.

Fixes #573

How it was tested

• Unit / synthetic tests:
  • make test-audio-mix: PASS (verifies channel counts, ITU 5.1 downmixing, 5.1 pass-through channel ordering, 3D binaural ITD delays, head-shadow filter, LFE low-pass filter, and soft limiter).
  • make test-pc-controls: PASS (verifies audio_output settings persistence and clamping).
  • python3 tools/i18n_check.py: PASS (0 missing, 0 unused across all 13 locales).
  • git diff --check: clean (no whitespace or formatting errors).
• Build:
  • make -j8 on Windows (MSYS2 UCRT64, clang): builds cleanly with zero warnings.

Checklist

• Based on the latest test and opened against test
• Built and run locally, and checked in the game
• No game files, keys, firmware, generated game code or save files in the changes

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[Revivedx]

Thanks, this is in test as a04bafd. We tested Stereo and 3D Headphones with headphones. We have no 5.1 speakers here, so the option's description says Surround 5.1 is untested on real speakers; a report from a real 5.1 setup would let us remove that note. Closing since test gets one clean commit per batch; you are credited as co-author.