TurboDelgado suggested the bard have an option to completely fail at the duet. It was an awesome idea and pretty easy to implement, even without additional voice options. Still all very experimental, but I think it really helps give this feature an added dimension of joy.
I changed the pitch and tempo, then rearranged portions using a cut-paste method to make the tracks. The result is pure, wonderful crap. William S. Burroughs would not be proud. Oddly enough, this would be the first shitty bard in the game, as it occurred to me that Lurbuk the Orc Bard can play as well as anyone, even if his singing is a little off.