You actually have to have the multi-track recording. In other words, the recording with each instrument and vocal on its own track.
Most commercial recordings, which I suspect is what you are trying to remove vocals from, are “mastered” recordings wherein all the various tracks have been mixed and mastered down to the consumer’s stereo or 5.1, etc., version. Most times, it is all the instruments and voices mixed to the left/right stereo channels.
So the only way you can do this, is to find a plug-in somewhere that isolates based on frequencies — one that that has been optimised to recognise the register of the human voice. I am not aware of one right at the moment but I am sure such a thing exists. I have never had to do this, so I haven’t bumped into it yet but I know it’s out there…
Best regards,
Ron Lindeboom
creativecow.net