This conversion is actually impossible.
A WAV, MP3 or AAC file is a digital music file, while a MIDI file is information on pitch and length of notes, which is then played by the MINI instruments on your computer. That's why a MIDI file will sound slightly different on different computers - the MIDI sounds they have installed might be different, or are different, for example, between versions of Windows or the MIDI instrument hard-wired onto the sound card. (MIDI = Musical Instrument Digital Interface), think of those old MIDI keyboards you can connect through the dedicated MIDI interface on older PC's (we have USB now).
There are programs that can 'listen' to MP3's and try and create a MIDI file from it, but in most cases it will sound terrible and nothing like the original, unless the MP3s is very basic. Some classical music actually works really well.