rename or prename is Perl a tool which can rename multiple files using regular expression.
For example to rename ??.mp3 files to 0??.mp3
rename -v 's/(\d{2}).mp3/0$1.mp3/' ??.mp3
-v, --verbose
-n, --no-act
Test before action
Usually you should use -n and -v option to test your expression. After you make sure that it works as you expected, remove the -n option to do the real job.