Issue could be anything from the EPROM that contains the audio data, any of the chips and capicators in the system it runs through, to the amp. Possibly the PSU is no longer supplying 12V which would cause this issue. Speaker could be unplugged/wires damaged.
Realistically if it's not something very simple, and you have no repair gear, you're not going to be able to fix it on your own. Still, it's a working cabinet, the game can always be replaced or repaired by someone else if it's the issue, and the price is good, go for it.