well, I do know of a program that allows you to mix the sound, masking (or even eliminating) vocals of certain songs.
I believe it's called DART Karaoke Author. It allows you to edit the sound and get rid of the vocals. Does not work with all songs though.
If all else fails, there's a website or two that have the songs that are already made without vocals. The karaoke mixes, I believe.
We can only grow the way the wind blows
On a bare and weathered shore
We can only bow to the here and now
In our elemental war
- Rush, "The Way The Wind Blows"