When a DVD or CD plays the sound is scratchy but if it is copied to the computer the sound is perfect what is the problem?

Cheap, noisy, equipment, or some other device in the vicinity of the cd/dvd player that is injecting unwanted frequencies into the electronic circuits of the player. Sometimes, it can even be the amplified speakers on the computer itself causing the problem. Half wave rectification will induce all kind of interferences into the sound system.