Did the sound problem begin when you installed NVRAM or when you moved the game?
Try something not invasive first... Sever the IRQ from the sound board. Easiest way to do this is to only use pins 1-32 on the J5 connector. If you have 34 pin plug, shift the connectors over two pins to the right on both ends of the sound board ribbon. That will leave 33 and 34 disconnected.
The IRQ is not required for the 6840 chip on the sound board and it being connected occasionally causes problems.