What causes popping sound from audio receiver speakers it pops when you change channels on tv and it pops when you shut the receiver off have optical cables feeding the avr from the cable box and DVD?

A loud pop in the speakers is one of two things. 1. DC voltage from the amp instead of the normal AC. (If this is the case the amp is slowly burning the voice coils of the speakers.) 2. a large spike to the input of the amp from the cable box (this would be less common, but possible.) In either case the protection circuit of the amp should trip and this should never get to the speakers. The cause is not an ac problem from the wall. The problem is probably in the amp itself. Do yourself a favor and get a stereo tech check it out or get a new system. But if you let it continue you will not have any speakers.