If you are not familiar with Microsoft Songsmith already you should check it out. Its a kind of reversed karaoke. You sing a song and Songsmith automatically generates the music suited for it. In this demo it gets clearer.

You can try Songsmith yourself here.

It might not be the most useful tool for a pro. Though I have to say the clips below are very funny. They used songsmith with the vocals from known hits. First out Motörhead - Ace of Spades!

Michael Jackson - Billie Jean

Beastie Boys - Intergalactic