The SDL_mixer-FLAC project is actually a patch that I have created for the current SVN checkout of the SDL_mixer library. SDL_mixer is a supplemental library for the Simple DirectMedia Layer library. The point of SDL_mixer is to add support for various audio formats that aren't available in the SDL core. By default SDL supports basic waveform sound, SDL_mixer adds formats such as MP3, OGG, and others.
This patch is an attempt to add support for the Free Lossless Audio Codec, also known as FLAC. It has been tested on the Linux platform and is currently being tested by upstream to be included in future releases of the SDL_mixer library.
Although the patch was originally written for the SVN checkout of the SDL_mixer library (which at the time of this writing pertains to the future 1.3 release sequence), it has been tested and also can apply to the SDL_mixer-1.2.8 library release (and plausibly future releases if they do not include the patch already).
As of 2008-02-26, this patch has been officially included into the SDL_mixer SVN tree. It will be included in future releases of the SDL_mixer library.