ALSA (Advanced Linux Sound Architecture): An audio library (and kernel level API) for Linux.
BruteFIR: A software for applying long FIR filters to multi-channel digital audio, either offline or in realtime.
Digital Room Correction: DRC generates digital room correction FIR filters to be used within HiFi systems in conjunction with real time convolution engines like BruteFIR. [Aladdin Free Public License (AFPL)]
DMM_Volume_DLL: The control of volume is realized by the u32 library. The library can work in all the MS Windows versions (95, 98, ME, NT, 2000 and XP). [Commercial]
DUMB - Dedicated Universal Music Bastardisation: 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]
Ethan Brodsky's Sound Programming Libraries: Sound libraries for the PC-DOS platform.
FMOD: A sound system on Windows, Linux and Windows CE, PS2 and XBox.
GSI (General Sound Interface): 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]
Imtelephone.com: Provide the audio/video codec Win32 static linked library with source code implement ITU G.722, G.723.1, G.729a, G,728 which support multi-channel concurrent. Also provide the audio arithmetic such as AEC, VAD, AGC, VoiceXML. [Commercial]
OpenAL: A free (LGPL-ed) and open source, cross platform audio library used for 3D and 2D sound.
Spark: A sound engine for Windows. It provides powerful sample, streaming, and module playback functions and can be used with and without DirectSound. It distributed in two forms: static (.lib) or dynamic (.dll) libraries.
Virtual Sampler SDK: Provides applications running on Pentium-class PCs under MS Windows 9x/NT/2000 with a MIDI-controlled, low-latency, fully programmable wavetable module implemented entirely in software. [Commercial]