Artist often release "albums" containing 3 versions of some songs - The actualy song, JUST the lyrics and then JUST the music. But I would have to agree with jm1234567890 on finding a program that actually did it and did it effectively.
Most plugins for Winamp (and probably OpenMG) work by doing the job of the equalizer. Common frequencies were vocal sounds usually occur are cancelled or have their volume reduced to create the effect that the vocals are gone. This always has effect on the music as well.