Q: Fading volume levels- in the same song?

I'm having a problem with hearing various tracks on iTunes, as somehow the volume fades in and out? This is happening on mainly soundtracks I have purchased on itunes, where more than one person is singing, so for example, I can hear person A just fine, but then when person B starts singing, I can barely hear them, and the sound is really tinny.

I have never had this problem before, and the tracks all used to play fine. I have tried uninstalling them, then re-downloading, but the same thing happens!

Any ideas?

Thanks!