Quoted from freeplay3:

The volume control is a new one for me too. My guess would be the coin door volume buttons were either not working or shorting out causing the volume to go up or down on it's own. They probably bypassed the door and put the control on the board for the volume control.

Quoted from LTG:

I'd check the power to each pin on the big connector on the DMD, if that is good. Then you might have an issue with the CPU, DMD driver board, or ribbon cables from CPU to DMD driver board to DMD.
You could have battery damage underneath the battery holder. And the battery holder could be bad. Use a meter and see if battery power is getting onto the board. All batteries should have the "+" end up.
No idea on the sound hack.
LTG : )

Quoted from LTG:

The back of the manual has the connector information and each wire color to each pin.
If you need a manual you can download it from here - https://www.ipdb.org/search.pl
LTG : )

Quoted from DumbAss:

J507 is not installed at the factory. There appears to be an external potentiometer for volume level control installed. Pin 2 connects to the MC3340P attenuator. Pin 4 connects to ground.
Someone has replaced U2 (MC3340P) and (fortunately for you) installed a socket. Someone has also removed U5 (x9503) and (fortunately for you) also installed a socket. However ... there's no IC at U5 so whatever volume level the CPU is trying to set is not making it from the CPU to the analog output. Your system appears to be dependent on the external potentiometer.
The focal plane of your pictures are not on the board itself so it's slightly out of focus but looking at it would force me to conclude that I would need to double check all the prior work done on U2 and U5 (verify continuity of all pins on both sockets).
If you want to restore it to factory you should remove the wires at J507, check the continuity of all the prior work done on the board and install U5 (x9503).
That's all I can tell you from the pictures you provided.

Quoted from DumbAss:

The schematic says that pin 1 is not connected and pin 3 is the key. On a real board neither pin 1 nor pin 3 are electrically connected to anything (no traces on front or back of the board) so you could connect the wire to either of those points but it would serve no useful purpose.
You would need to look for clues as to where something may have been soldered and has somehow come off from that connection point. Unfortunately your picture is not in focus. If you want to take some more pictures (in focus and well lit) and post them here I can take a closer look at them. It's probably easier if you just look and see in person.
It looks like the rail around the side of the board might be the point where it came off (a reasonable amount of solder in the exposed area) but that's just a guess as I cannot see it clearly. If you can fit the wire into this (like a jigsaw puzzle piece) that would provide confirming evidence that this was the point.

Quoted from LTG:

The reason for the potentiometer is your CPU has some serious damage. The battery damage gets the diagnostic buttons first, which would include your volume issues and why someone hacked in the potentiometer.
LTG : )

Quoted from LTG:

Contact Chrishibler here on Pinside.
He may tell you it would be cheaper to get a new board, depending on what your board all needs.
LTG : )

Quoted from DumbAss:

The schematic says that pin 1 is not connected and pin 3 is the key. On a real board neither pin 1 nor pin 3 are electrically connected to anything (no traces on front or back of the board) so you could connect the wire to either of those points but it would serve no useful purpose.
You would need to look for clues as to where something may have been soldered and has somehow come off from that connection point. Unfortunately your picture is not in focus. If you want to take some more pictures (in focus and well lit) and post them here I can take a closer look at them. It's probably easier if you just look and see in person.
It looks like the rail around the side of the board might be the point where it came off (a reasonable amount of solder in the exposed area) but that's just a guess as I cannot see it clearly. If you can fit the wire into this (like a jigsaw puzzle piece) that would provide confirming evidence that this was the point.

Quoted from LTG:

Try it in a different game, driver board too.
Then you'd know if all the issues are from the CPU or not.
LTG : )

Quoted from LTG:

https://www.greatplainselectronics.com/ Ed will hook you up.
LTG : )