Verified answer

MP3s are already heavily compressed. The more you compress it, the lower the quality. I suppose you could always lower the quality further or make it a mono track (instead of stereo).

You can convert mp3's to lower bit rate such as to 128 bit rate which is pretty much standard for mp3 quality. Also, you can convert to ogg vorbis and your file size will be half that of mp3 at the same sample rate without loosing quality. Good luck.