You can use the freeware sound editor, Audacity 1.2.6 (not the newer beta), and drop the mid-range frequencies in the equalizer. This is the range that covers most of the vocals. It's not a perfect solution, but might be good enough for what you're trying to achieve. There are also other methods like reversing the left/right channels. The software Dai mentions will use these techniques in combination with other filters. They work quite well, just don't expect perfection.
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