Missing sound of instruments/voice/ on CD tracks

I'm missing sound of some instruments or voice when playing CD's. Even duplicate CD's, when played, I'm missing some digital information or not having it picked up some where. For instance, the lead guitar part in a song may be totally missing or on other artists' recordings barely audible. It can be voice, horn, reed, string, etc., but it's always the same instrument even when played on another copy of the same CD. The following equipment is used: Gamut CD3 (2years old), NAT Symmentrical (Tube) pre-amp (3 years old), NAT Generator Mono block Amps (Tubes) new, Gamut Phi7 speakers (maybe 2 years old), various power cords of middle high end, and Acoustic Zen IC's Silver Reference II and Absolute (about 2 months old). Speaker cable is 5 years old, forgot the brand, and are double barreled. The speakers are bi amped. I don't know where to start in my trouble shooting before I call a audio shop for repair help. Maybe the laser in the CD is dirty? HELP. Thanks