Important point. If it tracks the volume control, then the problem is before the VC in the audio chain. If the volume doesn't change, then the problem is past that, which is mostly power amplification.


A subtle point about measuring the speakers and ground is to disconnect all speakers from the receiver, then measure for resistance from the speaker wires to a power ground. The resistance should be infinite. If not, one of the speaker wires is shorted to the ground somewhere, and this is likely to cause buzzing that doesn't go up and down with the volume.

All the other advice is good.