Ribbon cable connected between game board and sound board? Check!
Power cable connected between game board and sound board? Check!
Volume pot connected to main board? Check!*
Jamma harness connected to pcb? Check!
Cab powered on? Check!
Mortal Kombat running perfectly with glorious mono sound? Ch....Hang On.
What's that loud humming over faint sound effects? Nooooooooooooooooooo!
* The volume pot didn't come with the pcb but it is easy just to pick one up and wire it to the board. Which I did.