honestly im not sure if the pcb has a volume control on it, however my taito cab on the back has a volume knob attached to a mini pcb. not sure if the Gun.Smoke PCB has a volume control on it directly. I noticed that 2 wires were soddered into the volume mini pcb, and it has a molex with 4 wires comming back out. Only 2 wires are going to the speaker which i would guess is positive and negative, which have been cut. The 4 wire off the molex from the mini volume control knob are also cut. and one of the soddered wires on the volume control pcb is also unattached.
right now i just tried messing with it and the speaker is making a thumpering continious popping noise. The volume knob works because as I adjust it the thumpering / popping noises gets louder.