The sound board is the board on the left in the tender, but there is also a power supply board if it's a modular setup. Are there two boards plugged into a small motherboard? Can you tell us the part numbers silk-screened onto each board?
My method would be to try the boards in a working locomotive first, just to rule out some simple wiring issue. Have you checked the tether to see if it's still whole? The tether connection to the board on one end or the other is a common failure point.
