If you have an audio editor, you could cross-cancel the frequencies on the left and right channels (assuming its stereo), as vocals are usually the same frequency on both sides, whereas instruments would be different (unless its orchestral stuff, anything besides rock, for example, Creed cancels out almost perfectly, whereas TSO wouldn't stand a chance). Theres also about a 90% chance that theres a filter on the program that's called "vocal cut" or something like that, which does the same thing. Save as whatever you want. This will cut the vocals, but it's completely lacking bass (because bass tones are frequent enough that they could be coming from more than one instrument). To fix this, make a new copy of the original song, try to cut everything out except the bass, and merge the two files together. If you're lucky, you have yourself a song sans vox.
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