The latest version of SDL_sound can be found at:
https://icculus.org/SDL_sound/
Note that SDL_sound is not an official part of the SDL project, despite its name.
The basic gist of SDL_sound is that you use an SDL_IOStream to get sound data into this library, and SDL_sound will take that data, in one of several popular formats, and decode it into raw waveform data in the format of your choice. This gives you a nice abstraction for getting sound into your game or application; just feed it to SDL_sound, and it will handle decoding and converting, so you can just pass it to your SDL audio callback (or whatever). Since it gets data from an SDL_IOStream, you can get the initial sound data from any number of sources: file, memory buffer, network connection, etc.
As the name implies, this library depends on SDL: Simple Directmedia Layer, which is a powerful, free, and cross-platform multimedia library. It can be found at https://www.libsdl.org/
Support is in place for the following sound formats: