This is a weird one. On a few songs (and only on a few), the background vocals are very weak. An example is the female background vocals in Frank Sinatra's "That's Life." But also, the lead vocals are weak in the Beatles' "When I'm 64." This is only happening on a few songs and only on our big system. The same CDs sound fine on other CD players. I've tried adjusting everything and tried cleaning the CD heads, both with no change. Any idea what the problem could be?

Scott