I have the Night at the opera 30th anniversary special (one with the 5.1 surround mixes)

i've managed to extract a vocal track or two, but i've read that i can use it to cancel out the vocals in the song it was ripped from (providing it's the same session)

i'm using Adobe Audition 3.0 but i will download something else (i.e Audacity) if that's what's required.

help anyone?