Do you mean to have a single MIDI note trigger playback of an audio file? If so, you can do that with the built-in ReaSamplOmatic5000 sampler.
If you meant to convert an audio song into many many MIDI notes (though you said "single note"), it's best to do that by ear and programming data tailored to the particular virtual instruments and samples you have.
Melodyne Editor can generate MIDI from polyphonic material, but you probably won't get ideal results from processing an entire song with different instruments. It's designed around single polyphonic instruments like a piano or guitar.