Short answer: you can't do that.
Longer answer: There is a way that seems to work with a few songs. Split the two stereo channels, invert one of them and add then add the two together. If the vocals have been added equally to the left and right channel, left and right vocals should then cancel each other out, leaving a mono music track behind. But it really only works if the song was mixed with vocals split evenly - for all other music, I am afraid you are out of luck.
- Martin