PortAudio is a free, cross platform, open-source, audio I/O
library. It lets you write simple audio programs in 'C' that will
compile and run on many platforms including Windows, Macintosh
(8,9,X), Unix (OSS), SGI, and BeOS. PortAudio is intended to
promote the exchange of audio synthesis software between
developers on different platforms, and was recently selected as
the audio component of a larger PortMusic project that includes
MIDI and sound file support.

PortAudio provides a very simple API for recording and/or playing
sound using a simple callback function. Example programs are
included that synthesize sine waves and pink noise, perform fuzz
distortion on a guitar, list available audio devices, etc.