I tried doing this a long time ago and ran into two blocks. The first one is that for good conversion, you need to know all the instruments used and what they will convert to in MIDI. the second is worse: many of the effects used in mp3s come out as static high notes in MIDI. So, there really isn't a good one. If you had a project file of an mp3, you could easily export that as a MIDI but not the mp3 itself.