The ScratchLive.net forums play host to a ton of musical knowledge and you should take full advantage of it when you can.
This post in particular is super useful. In it, he uses Adobe Audition, but any music-editing program should do the trick.
Here’s what poster D-Twizzle says:
Rip the album version and instrumental version from CD as a standard Uncompressed WAV file.
You should have 2 files which are normally the same length and size.
Open up Audition and line them up PERFECTLY. Zoom in and make sure the wave forms peak at the exact same spots. This part is KEY.
Play the tracks, they should play as normal, maybe a little loud, since you could be peaking past 0db.
INVERT either file, doesn’t matter which one, but only choose 1 file.
Now when you look in the zoomed multi-track view, the wave files should peak at the exact opposite spots.
Now play the tracks, and you should just hear the vocals/acapella. Export that to a new file and save it as your acapella.
Why does this work? Since the wave form phase for the instrumental and Original version are exact opposites, they cancel/phase out each other and you are left with the vocals.
Not sure if this works, but it may be a pretty handy little trick.