Wozza (hubby) is thinking a whole new board replacement.....???

Its unlikely you need to replace the sound board   As wiredoug suggested start by checking connectors etc, in particular check the connectors from the MPU to the sound board are connected correctly.

Galaxy is a great game that gets a lot of love here on AP. I have one in my collection in need of a playfield restore.. one of these days i'll get to it.