2010 Hardtop S, Standard Sound (Boost CD).

Last month I suddenly lost all sound. The radio still lights up and appears to be working perfectly. The radio will still scan and lock onto stations, and all functions work. I am also getting no system sounds like seatbelt dings.

After some research, it was recommended to replace the radio. I purchased a used radio from eBay, and just installed it. The new radio has the exact same issue... no sound.

So far this is what I have checked:

1. Replaced head unit with used (could be a 2nd bad radio?)
2. Verified all fuses are fine.. pulled every fuse in the car and tested it
3. Checked the speaker wires with a multimeter. All resistance measurements where similar across all 4 pairs of speaker wires.

I am at a loss of what to check next... do I get a 3rd head unit and try that? This is the base system and doesn't have an amp anywhere that I know of.