check the eproms and their sockets and your yamaha chips ym2151 and ym 3012. they produce the music. Could be as simple as chip creep

that is your knocker. It hits when you get a free game etc. If you have bad hum make sure all the screws are secured nice and tight on all the circuit boards. Very common to have them back out or go missing. Assuming all are good and tight...check wiring and connectors followed by a recap of the sound board .

Oh yea if all the screws are tight on the boards and you have hum. With the power off ........ back the screws off on the sound board by 1 or 2 turns and retighten them all back down . I do not know why...but sometimes it works. Barring that caps are common to fail as they are all getting quite old and way past their useful life.

you only need the wires soldered to the speaker. That extra plug goes no where and should be cut off. The other wires laying there go to the knocker coil. It knocks when you win a free game or win a special etc... Not critical to the game.
If you are missing screws than there is your problem. Replace them and play away. They are #8 by 1/4 screws. Make sure you check them all there is a lot of them.

No worries...glad to help anytime. Have a game on your machine for me! #8 x 32 x1/4 inch long machine screws. Regular steel is fine.
All your boards are grounded through the common ground plane which is the backbox metal plate....Missing or loose screws...problems arise or at the least ground hum.
Get your screws and install and