Free Audio, Sound, Music and Digitized Voice Libraries

APEG
APEG is an audio/video add-on for the Allegro game programming library (see the Free Games Programming Libraries and Source Code page for more information on Allegro) that allows your program to play MPEG-1 and Ogg streams in the same way it plays FLIC files.
New Audio Components for Delphi
New Audio Components, or NewAC, supports the playing of audio files of a variety of formats, including wav, Ogg Vorbis, FLAC, Monkey Audio, WavPack, MP3, and WMA. Sound files need not be external, but can be embedded in a program's resources. Other features include the ability to acquire audio data from input devices like microphones or CDs, the facility to encode audio into the supported formats, mixing, concatenation, cutting and insertion of audio streams, audio resampling, etc. The code may be used for both open source and commercial use.
TurboPower Async Professional
If you need to add speech to your programs, or enable it to process speech input, TurboPower's Async Professional provides you an easy access to Microsoft's Speech API (SAPI) allowing you to integrate voice/speech recognition and speech synthesis in your programs. It also handles the sending and receiving (as well as viewing and printing) of faxes (and converts BMP, DCX, PCX and TIF files automatically), alphanumeric and text (GSM/SMS) paging, high speed file transfers (including the major file transfer protocols like Zmodem and FTP), IP telephony (audio and video, Voice Over IP or VOIP), Internet communications (Winsock access), supports ISDN modems, RS-232 and RS-485 standards, and includes an XML-based modem database. And so on. Like most of the TurboPower tools, this open source toolkit may be used with Delphi and Borland C++ Builder and ActiveX environments like Microsoft Visual C++ and Visual Basic.
TMidiGen
This Delphi and C++ Builder component allows you to create MIDI sound effects and note sequences in your programs. It generates the MIDI data in memory, and thus does not require external files or resources. It has 175 instruments, Advanced Pitch Blend, Modulation, Sustain, Chorus, Reverb effects, volume and pan adjustments, etc. It appears to be free (although the website makes no mention of the licence under which it is released; let me know if you have information about this).
TToneGen
TToneGen is a Delphi and C++ Builder component that lets your application create waveform sound effects. It generates the WAV waveforms in memory. It supports amplitude modulation, full AHDSR envelope shaping to alter the dynamics of the selected waveform, handles a frequency range from 20Hz to 20kHz, etc. It appears to be free (although the website makes no mention of the licence under which it is released; let me know if you have information about this).
PortAudio
This is an open source cross-platform audio I/O library. Using this library purportedly allows your sound source code to be portable across Windows, Macintosh, Unix (OSS), SGI and BeOS. The API allows you to record and play sound. Linux support is currently in BETA.
libsndfile
This is a C library that allows you to read and write sampled sound such as using the Windows WAV format, the Apple/SGI AIFF format, Dec AU format, RAW headerless PCM files, etc. It is released under the GNU Lesser General Public License. The library can be compiled and used on Windows, Linux, Macintosh, and "just about any Unix".
BASS Sound System
BASS provides developers with a set of powerful yet easy to use sample, stream (eg MP3, WAV, custom generated), MOD music (XM, IT, S3M, MOD, MTM), MO3 music (MP3 compressed MODs) and audio CD playback functions. It also supports 3D positioning of samples, streams and MOD music with support for Aureal's A3D and Creative Lab's EAX (Environmental Audio eXtensions). It requires DirectX (version 3 or above) and uses DirectSound and DirectSound3D hardware accelerated drivers and MMX CPUs. API for C, C++, Delphi, Visual Basic, and TMT Pascal are provided. Note that it is free for freeware but you have to pay if you're developing shareware or commercial applications. Only Windows 9x/NT/2k platforms (ie, Win32) are supported.
Soundlib2 Sound Library
SoundLib 2 is a library for mixing and playing sound files under Borland Pascal 7 and the Free Pascal Compiler with 32 bits DPMI on a SoundBlaster or compatible. It currently supports WAV, VOC, RAW, MOD, S3M and XM files. The library does automatic sample rate conversion, sample interpolation, has 32 bit mixing routines with support for a maximum of 256 channels, and support for Dolby Pro Logic. It comes with a Borland Pascal help-file and some demo files.
OpenAL: Open Source Audio Library
OpenAL, the Open Audio Library, purports to be a "joint effort to create an open, vendor-neutral, cross-platform API for interactive, primarily spatialized audio." Members include Creative Labs and Loki Entertainment Software.
Open Source Audio Library Project
The Open Source Audio Library Project (OSALP) is a library with a set of C++ classes that handles audio functions. It is multiplatform. It currently supports the MP3, WAV, au, aiff, aifc (etc) formats. It also has several worker classes: audio sample rate converter, audio pitch change, audio test source, audio editing, audio timer recording, audio VU meter and spectrum display, audio mixing, reading and writing audio data to devices or files. Source code is available, and the project is in the initial stages.
SMIX Programming Library
The SMIX is a sound programming library allows your program to play up to 8 simultaneous sound, mixed in real time, on any Sound Blaster (or compatible) card. Although it is free, they request "a contribution" if you use it for commercial purposes. Versions are available for Turbo Pascal (real mode), Borland C (real mode), Watcom C (DOS/4GW protected mode), and TMT Pascal protected mode. The site also provides links to third party ports for Microsoft C, Quick C, Microsoft Visual C, and Borland Pascal 16 bit protected mode.
SBVOX Programming Library
SBVOX is a sound programming library that allows you to play VOC digitized voice files in your programs via Creative Labs' CT-VOICE driver. The Turbo Pascal source code is available.
SBDSP Programming Library
To quote from their website, SBDSP is a sound programming library that "plays VOC digitized voice files directly on a Sound Blaster or compatible sound card without using Creative Labs CT-VOICE driver." The full source code, in Turbo Pascal, is available. The library is no longer supported.
VDMSound
This item should probably not be listed on this page since it's not a programming library or the like. However, if you develop or maintain DOS games that need to run in a DOS box on Windows NT, 2000, XP, you might find this soundcard emulator useful. It emulates an MPU-401 interface (for MIDI), a SoundBlaster compatible soundcard (SB16, SBPro 2, SB2, SBPro, etc) and a standard game-port. The beauty of this emulator is that independent of the user's audio hardware; it works with any soundcard and even on systems that do not have a soundcard. The emulator is free, released under the GNU General Public License and comes with source code.