I have seen some sites on the web (www.mp3projects.com) that introduce hardware projects concerning
mp3 decoding.
There are stand alone mp3 player projects,
and pc based.
These projects are based on existing mp3 decoder not expencive chips as STA13 and MAS3507.
As I can see some good hardware & software
atari freak could do atari (st,tt) mp3
project.