I asked almost this same question a while back.

The Firepower and Black Knight I used to play at the arcade in the 80's had a box speaker attached to the top of the back box and sounded awsome.

I posted if anyone knew about this upgrade, as I would love to do it to my current Firepower and Black Knight, and basically got no response....and most said it was never done.

I later came across a picture of me in the arcade, with both Firepower and Black Knight in the background with these speakers on top of the back box.

Would still love to know what is needed for this upgrade.