It's not possible. Many people have asked, but it's just not possible. If you have a song with the vocals panned hard left or right, all by themselves, then yeah you can do it (sort of). But all finished songs are mixed down, with all the instruments and vocals mixed together. The only way to even attempt to remove the vocals is through filtering, but that will just mess up the whole sound, and you'll still have the rest of the sounds in there somewhere.