- OSS (Open Sound System)
- ALSA (Advanced Linux Audio System)
- lame (mp3 encoder)
- libmad (mp3 decoder)
- FFMPEG (an audio/video decoder/encoder super-multi-purpose lunch pack)
Wikipedia is a friend, although doesn't go deep enough: http://en.wikipedia.org/wiki/Digital_signal_processing
- MusicDSP (look up the mailing list for a lot, lot information and implementations)