Yep as MartyJ said
1. As Rod71 said I'd check the coin door switches (mainly the + volume one), do they move freely or are they sticky from stuff being spilled? They can be cleaned by soaking in metho or alcohol.
2. Then as Nug suggested and this is straight from
http://www.pinrepair.com/wpc/index3.htm#soundVolume up FULL and Can't turn it Down.
The volume control on all WPC games is electronic. On pre WPC-DCS games, this is controlled by an electronic prom pot. This E-pot is a X9503, at location U5 on the sound board. If turning the volume up or down has no effect, and the volume is stuck on full blast, this is the first component that should be checked. Also the capacitor C18 (47 mfd, 25 volts) that connects to the E-pot can fail too, and should be checked. As described above, the electronic volume control can be disabled by removing A-12738 sound board jumper W9 to disconnect the software controlled volume circuit. Then connect a potentiometer (any value 5k to 200k ohms should work) to connector J507:
* J507 pin 2: To center pot leg
* J507 pin 4: To outside pot leg (Analog ground)
