These *is* a technique for removing the vocal from a song.
It doesn't always work, and it relies on the fact that the lead vocal track is usually placed dead centre between left and right - i.e is in exactly equal amonuts and in phase on the L and R channels. Meanwhile all the accompaniment tracks are panned more or less off centre. So what you do is subtract R from L and with a bit of luck the lead vocal will cancel out and you'll get a (rather bad) mix of the backing only.

Any reverb added to the vocal will not be removed by this technique.

If you have a way of doing this and varying the proportions of L and R, you may be able to change the mix in ways that help to hear the individual parts better. You can't separate out one voice, but you can perhaps almost eliminate one voice at a time which would give you three instead of four to sort out aurally.

In a natural stereo recording made with two mics, this may not work at all well because of phase differences that mean simple subtraction doesn't cancel things out.

You certainly don't have the full multi track information on an ordinary stereo CD.

Anahata