Free Sound and Audio Libraries
OpenAL
"OpenAL is a cross-platform 3D audio API appropriate for use with gaming applications and many other types of audio applications. The 1.0 specification has been complete for quite a while now, and the Linux, MacOS (8/9 and OS X), and Windows (all 32-bit versions) versions are fairly mature."
Open Source Audio Library Project
"This is a project designed to implement a world class set of classes in C++ that will handle all of the audio functions one would like. It is designed to be multi-platform with UNIX based platforms as the base. This project is still in the beta code phase and a beta version that will illustrate the power and flexibility is now available. This version supports the Linux (OSS) audio device, Solaris Sparc audio device, FreeBSD (OSS) audio device, wav, au, aiff, aifc, mp3, and numerousother formats."
PortAudio
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.
Libsndfile
Libsndfile is a C library for reading and writing files containing sampled sound (such as MS Windows WAV and the Apple/SGI AIFF format) through one standard library interface. It is released in source code format under the Gnu Lesser General Public License.
SBDSP Programming Library
"SBDSP plays VOC digitized voice files directly on a Sound Blaster or compatible sound card without without using Creative Labs CT-VOICE driver. Sounds are played in the background, using a fraction of the CPU time."
SBVOX Programming Library
Freeware programming library that plays and records VOC files in the background on a Sound Blaster sound card using Creative Labs' CT-VOICE driver. Turbo Pascal source code is included.
SMIX Programming Library
"SMIX is a freeware programming library that will play up to 8 simultaneous sounds. The sounds are mixed in real time and played on any Sound Blaster compatible sound card. It provides transparent support for a variety of sound cards and will used auto-initialized 16-bit output if possible. A contribution is requested if SMIX is used for commercial for-profit use."
SoundLib 2b
32 bit mixer unit that plays WAV, VOC, RAW, MOD, S3M and XM files. See the website for more details - appears to be free for non-commercial use.