per sa(sp? per se
You're not going to be able to do it really well unless you have a recording in which the vocals were recorded on a single channel, which isn't really often the case these days, though it was more common in earlier decades (a lot of Beatles songs come to mind)... anyhow, if you do just need to get rid of either the left or right side, most any sound editing software you download should be able to do it for you. I googled "sound edit mac download" a few weeks ago (I was actually trying to do the same thing, though it was so I could make some ringtones for my cell phone!) and came up with quite a few programs; nearly any of them would have worked; I've unfortunately since erased them all but you shouldn't have trouble finding stuff...
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