My radio is working, in my 07 LT HHR, just can barely hear anything from any of the speakers, front or rear, left or right. Seems like a poor plug connection somewhere.
Any ideas where to look first ???
The roads here are atrocious and are causing things to loosen up. All driving here is avoiding potholes, everyone looks like they are driving drunk, its ridiculous !
Anyways, would seem that I have a poor main speaker control connection as its happened suddenly to all speakers and I don't know where to look first.

 

I'm going to guess the foam on the speakers is dust. Mine were, new drop in's and great sound again.

 

If it did happen to all the speakers at once, then most likely it's not just a "bad" speakers problem.
From your description, sounds like a radio, wire harness or ground issue. Before you have to pull the radio for checking, try this:
Move all the controls (balance,fade, volume etc.) back and forth through all their ranges. You could have one that has hit a bad spot.

Fwiw. Your speakers are 8 years old. Don't be surprised if they're on their last leg in life. Even if you fix you're radio problem, the speakers may not have much life left.
The factory speakers aren't known to have a long life span.

 

Pioneers!