If you are starting with an mp3, there are all sorts of programs that will convert the mp3 to uncompressed WAV format. (I think ITunes and WinAmp can both do this) You can then edit the WAV file and cut it into smaller pieces using many different programs. (I use SoundForge, but there are certainly all sorts of free progams out there) Then you can recovert the smaller WAV file into mp3 using a typical ripper program combo like Exact Audio Copy + LAME.
I don\'t know of any programs that allow you to edit the mp3 file directly, but maybe there are some out there.