Hmm, this would be very hard to do indeed, since MIDI is a bunch of code signalling the sound card to produce sound, while WAV and MP3 are actual recordings.
If it were possible, the program would have to be able to decipher between instruments, notes, and then make a whole MIDI file based on this, which would be very hard indeed and almost impossible for a computer program to do.
It would be possible mind if the MIDI had to be polyphonic, in which case an MP3 or WAV could only have 1 instrument playing.
I've seen this one, that seems to do what it says, but I haven't tried it, and its not free:
I can't find any free ones though I'm afraid, since the complicated process of it seems to make programmers want you to pay for the code.
I've seen on the web though how many of these programs can't decipher, and so mess up the process.
If you want though, there are many many many MIDI files already on the web to download of popular music, etc that may help you instead, which you can drag straight onto the phone.
Just do a search on google for MIDI music.