I would suggest you take a ohm meter vom and check each wire from the battery connector to where it solders on the board, I have had one engine that had exactly that problem and I had a 2.9 ohm resistance from the 9 volt connector to the board and that was just enough that the sounds cut off immediately!! check both red and black wires one at a time and I think you will find your problem!! sometimes inside the nine volt connector comes loose but the insulation looks like the wire is still connected when in fact it is not, I wasted a lot of time pursuing that exact issue so check your wiring first before anything else ok and good luck!
Alan
