Asked and Answered. This type of audio editing is almost impossible! Some plugins claim to do this but the end result is severely destroyed vocals. I could give it a shot in Audacity, but I guarantee it won't sound very good. One possible way to do this is to have a karaoke version (minus vocals) and a studio version (with vocals) and invert them and convert to a mono track. If the karaoke version is a match note for note, beat for beat to the studio version; then when it gets mixed the vocals should be the only remaining track. But that example will ONLY work in a perfect World! If you'd like for me to give it a shot just PM me. I can't promise anything.