If you figure out how to expose the circuit board, I'm standing-back and standing-by to comment on what the problem could be. The exploded parts list shows this as the circuit board which is/was $25 but NOT AVAILABLE from Lionel.
If this is indeed what the circuit board looks like, it's hard to believe the foghorn sound electronics is independent from the background sound electronics.
But since this circuit board most likely drives the lighthouse motor/lights, there is some discussion as to the best way to restore and/or potentially enhance lighthouse operation.