Since you have other boardsets there, IMO I'd swap top boards and see whether the problem is on the top or bottom board. The top board does all the audio generation, but the bottom board does the final stage amplification to drive the speaker. Low audio could be a component around the TDA2002 on the bottom board, or an LM324 (or component around it) on the top board.
If all audio sounds balanced, but weak, I'd suspect it's either around U83 on the top board, or U122 on the bottom board. I don't have a board nearby to check, but I don't think the jumper wire between boards goes through any LM324s on the bottom board. If certain channels are weaker (easily checked in test mode), then it's almost certainly on the top board.
DogP