No sound on DMG - problem is in the back board

Hi.

I have a Gameboy DMG that has no sound on the speakers and very weak and dim sound trhough the headphones. I've been able to confirm - swaping the boards around with another DMG - that the problem resides in the back board (the one with the cart input), but I have no idea where exactly...

Do you have any input?

The board seems perfectly fine, all the capacitors are well conected and all the cables have conductivity... Is there one particular cap I should look at?

Thanks in advance!