you can also swap the sound board ribbon cable around end to end and see if it persists.
Seen a bad ribbon cable cause this exact issue . Do as Kbliznick says and reseat socketed chips like the eproms etc. Plus reseating the cable many times may help as it cleans the terminals off the more you do it.
Never hurts to get a new ribbon cable though. Cheap way to make sure its not the problem. You should inspect the pins on the boards too. Seen the odd pin missing before and some not making connection due to poor factory solders.