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An audio library (and kernel level API) for Linux.

A high-quality IT/XM/S3M player library. Versions 0.9+ will have MOD support. An add-on is available for playing OGG files. [Freeware]

A sound system on Windows, Linux and Windows CE, PS2 and XBox.

An audio system with network support and provides the ability to use stereo, 3D audio, doppler effects, music playing (MIDI, HMP, MUS), and cd playing and ejecting/changing. [Open source, BSD License]

A free (LGPL-ed) and open source, cross platform audio library used for 3D and 2D sound.