Diagnosing a doorbell fault
If it stopped after painting, then I would guess that it has nothing to do with what's behind the wall. (Unless they were able to get behind the walls themselves) Could be just bad luck, but I'd suggest checking the wiring connections on the bells. Take them off and reconnect them if needed.

Could your system be connected together at the bells?





After the comments below, I've (terribly) tried to draw up what it sounds like it looks like.


My best guess is that there used to be an old transformer that was used for the old bell and button. The contractor that added the new transformer would have removed the old one and used it's box, or wherever it was located, as the junction location to reconnect everything.. Does this sound reasonable? Since you have power to the light switch, but neither bell works, the issue must be between the button and the junction box or between the junction box and your new transformer. It could be a wire that's come loose in the box or something that's interrupted one of those two wires.

You might call the old contractor and ask about this possibility? If they did something like this, they should know where the box was.