Add a note about required audio format

[devnull42]

The instruction specify that audio files must be in OGG format. This statement needs to be expanded. Specifically, audio files must be in a OGG container, with a Vorbis audio codec.

I used VLC to convert existing files to a working format. The following transcoding properties worked for me:

$vlc -I dummy "$f" --sout='#transcode{vcodec=none,acodec=vorbis,ab=160,channels=2,samplerate=44100}:standard{access=file,mux=ogg,dst=out.ogg}' vlc://quit

Consider adding this note to the instructions.