you using xp?
put the mp3 files (or shortcuts to them) in
C:\Documents and Settings\**user**\My Documents\GTA San Andreas User Files\User Tracks
now, open up your game. load save file or whatever, then press esc, then go to options, audio options, then user tracks. then you have to say scan for user tracks. NOW, you will be able to listen. (use 4 to change radio stations til it says user track player)