I do a lot of multi track recording, but I'm no sound engineer. However, from what I do know, once tracks are mixed down you can only manipulate frequencies. And since the voice shares frequencies with many instruments I think it would be near impossible to get rid of a voice without getting rid of all but the bottom end bass and the high end cymbals. The voice tends to sit smack in the middle of everything.
Now if there is a program that can do this, umm yeah I'd like to know. Kinda like a Photoshop for sound. Nice idea.