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[Koen]

I can remember a way of how vocal removing was done. Vocals are seen as mono sounds, so sounds that appear in both channels (left + right) at the same time. So if you delete these common values from both channels you should have a song without vocals, thus only maintaining the stereo sounds. The problem is though, that nowadays there are too many effects used, like stereo enhancers, reeverbs etc, making it harder to remove these vocals.

adobe audition has a preset in removing vocals. It works quite well, you should check it out.