I just picked this machine up from a friend so I don't really know who was messing with it or what they were trying to do. That is where you will probably find the other wire for the speaker. That cicuit goes back to the motherboard behind the reels. It will not! You need a complete circuit for the speakers. The green wire in the photo that seems to come from the door looks like the genius who mad the resistor gang figured he could ground the speaker to the case and it would work.

He sells them on Ebay and he is a Pachitalk member.

Get rid of those resistors and get a volume control for the speakers.Check out FTKSERVICES inline volume controls. Run a lead from that wire (which you will hopefully find) to the speaker and reconnect the black wire to the speaker as well to form a circuit and give it a try! I would suspect that it is the other wire that should go to the speaker. I would recommend that you follow the thin black wire that is connected to the bottom of those resistors back towards the reels and motherboard to see where it comes from and look for another wire, probably cut, probably green or blue, that seems to have been paired with it. The gang of resistors you have connected to your speaker (picture 100_0549) looks like a nutty attempt to lower the sound by increasing the resistance, which a normal potentiometer would do easily!