MP3′ Tech calls this “the biggest MPEG audio source codes area avaible on the Internet.” Find source for MPEG-1/2/2.5 Layer 1/2/3, MPEG-2 AAC and MPEG-4, as well as UI code.

Reader Robert S. said It seems very hard to find a description of the Layer 3 bitstream format. Fimally found it here.

http://www.mp3-tech.org/programmer/programmers.html