MPEG.ORG is a roadmap to the best MPEG resources on the Internet.
This website contains pages with Video, Audio and Systems technical resources, source code, test bitstreams etc. There’s also have a page dedicated to MPEG Audio Layer 3 aka MP3. The MPEG Software Simulation Group (MSSG) page gives access to the source code of several public-domain MPEG encoders, decoders and players.