Unless it's a very simple single line of music you're asking a lot of any computer. Could you listen to say two guitarists jamming together and work out exactly which note is played by which player ? The human ear can often distinguish different lines in music by the pitch/tone/timbre of the voice or instrument, but much of the time we'd still get it wrong. A precocious young Mozart may have been able to write out most of the score to the "Allegri Miserere" after hearing it once, but he was an aknowledged musical genius - anyway the piece repeats about 5 times so it isn't quite as hard as it sounds. MIDI to WAV/MP3 is a doddle - it's just like recording a performer/group/orchestra playing from a score. Doing it the other way round is like giving a computer a JPEG image and asking it to name all the objects in it - it can be done, but currently a human being does it much better.