Zipping an mp3 won't reduce the file size further - otherwise they would've implemented it directly in the mp3 algorithm!
The only sensible things to do with reducing the sound size involve either shortening the track, compressing it further with mp3 or using MIDI instead.
or compress with lower bit rate mp3! Quality isn't as great, but for a game, is often not an issue; given the player will be more concerned with the actual game play.
Converting the track from mono to stereo can save you lots of space (if your original was a full stereo mp3)
Alternately, if you still want the stereo separation, re-encode it in Joint Stereo. That can save some space. (You save on audio information that is identical between L and R.. usually drum tracks, bass, vocals)
