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.