Mac seems to be in karaoke mode

I have a Mac Pro, dual quad core, 8 GB RAM, which I use mostly to play music from my iTunes library.

I notice that the vocals on many of my songs seem attenuated. In some cases I pretty much don't hear any vocals. I import the same CD into iTunes on my Windows PC and the songs sound fine. Both computers run to the same sound system through a mixer. I get the same effect with or without the mixer.

I expected my Mac to at least have a flat, uninteresting frequency response but this one seems permanently locked in some kind of karaoke mode. I've messed with the equalizer in iTunes but I can't bring the vocal frequencies up enough to make it sound good.

Is this normal? Can it be fixed? Can it be fixed without me sending the whole computer to Apple (despite my comments about only using it for iTunes I am an iPhone developer and can't be without this machine).

Thanks!!