The sound board has two paths to ground, the normal ground for logic stuff and then 43v to 12v reg has an isolated ground return. When there is grounding problems you get noise out the speaker.

A lot of times people can resolve this by backing the sound board up off the grounded backbox mount. Either leave the screw loose or put an insulator in. But ultimately its usually a ground problem related to connectors. Check rectifier board, backbox ground braid is attached to cabinet, connectors.

In the -next generation 51 sound board bally does not ground the board against the mounting bracket. They have the copper pulled back away from the screw points. I have wondered if it is because the earlier generation can have the symptom you describe.

When the game is moved and the head comes off some connectors get cycled and could expose connector problems. Find the rectifier board, I think it is still in the bottom right side of the head in this game. The grounds all come back to this board (sound buzz) and the playfield lamps bus voltage and GI return go this board. Give it a good visual inspection for burnt looking connectors.

The always on lamps are "GI" and should read around 6.3V AC

The computer controlled lamps are "switched illumination" or "feature lamps" and around 6.3V DC. With a multimeter you can see if your lamp voltage is missing.