midi to wav, or mp3 is one thing, but the reverse is near impossible... midi isn't an audio file... midi files do not contain audio information, they contain instructions on how loud (velocity), what pitch, and how long to hold it, and other such instructions... the sounds of midi are ROM samples within your sound card or audio device...
an app that would convert mp3 to midi would have to be able to analyse and separate different instrument sounds, and volumes and pitches, and would no doubt require quite a bit of processing time depending on the lengthe of the mp3 file being decoded, and my guess would be that even if someone has created a program to do this, it wouldn't be very good at doing it...