May 01 Hum a Few Bars, and I'll Fake It

Celemony Melodyne has one foot in audio, but the other in MIDI because the analysis that it runs on audio ends up being easily converted to MIDI data. If you can sing with consistent tone and level, Melodyne can convert your singing into MIDI. The same functionality for monophonic tracks exists in many DAWs.

MIDI data has been extracted from the guitar track at the top, and is now being edited in a piano roll view editor.

This has other uses, too. For example if you're a guitar player and want a cool synth bass part, you can record the bass part on your guitar, extract the MIDI notes using Melodyne's analysis (how you do this varies among programs, but it may be as simple as dragging an audio track into a MIDI track), transpose the notes down an octave, and drive a synth set to a cool bass sound. You may need to do a little editing, but that's no big deal. 

Here are some videos on how to do the same thing in our Platinum sponsor's DAW- Ableton Live. 

Audio to MIDI in Ableton

Here is a link to a more detailed article on how to convert Audio to MIDI in three different DAW-Ableton, Cubase and Sonar.