I would start with the edge connectors. With the power off, pull of the connector to the sound pcb and reseat it (pull it off, put it back on) and see if that clears it up. If not, reseat the other end of that cable in the driver board in the back box. If that doesn't work, pull the sound board and look for cold solder joints, then look at the driver board for cold solder. When the boards are out, you can 'eraser' the fingers clean to improve the connection.