'04 accord ex radio problem

Let me start by apologizing because I know this is going to be lengthy. My uncle recently asked me about his stereo in his '04 Honda Accord EX that stopped working. He said that the stereo would "pop" occasionally from the speakers and he would have to turn the unit off and back on. Then finally one day it popped, and there was no sound no matter what he tried. He drove it like this for several months with no sound. The stereo turns on, the CD player will accept and eject CD's, and the tuner seems to function, but there is just no sound. I told him there was probably an intermittent short in one of his speaker wires that possibly damaged the internal amp. I removed the stereo from the vehicle and tested all of the speakers by running a pair of wires from my garage receiver into the factory harness plug that connects at the HU. All speakers functioned normally. I disconnected and reconnected the unit several times and still no sound. I assumed the amp was damaged. I purchased and installed a used unit on ebay and it worked perfectly for 3 days. He then heard another "relatively small pop" from the speakers and once again, there's no sound. There was a 30 day warranty on the ebay unit so I sent it in for repair and they sent me a different unit back. In the meantime I installed his old stereo so that he didn't have a huge hole in has dash (plus the climate control is tied into the stereo) and after a few days his original stereo started working again. Now my uncle is afraid to use the stereo because he wants it to work when / if he trades the vehicle in. Does anyone have any idea what may be causing this problem, and why the stereo that stopped working would all of a sudden start working again? Any troubleshooting tips/ideas? Am I correct in assuming the amp is internal to the head unit?

It sounds as if you have a speaker that's grounding out.  Begin by disconnecting the speaker that the POP came from.Do it right the first time

That was my first instinct, unfortunately I don't drive the car, and I don't know what speaker the pop came from...from my uncle's description, it came from all speakers...but who knows.