3o. When things don't work: Sound Problems.

The sound on WPC games is very robust; it just doesn't fail too often. But here are some things that do fail related to sound:

Re-seat all the sound board ribbon cables. Surprisingly, this fixes a large number of WPC sound problems!
Bad rectifier diodes on the sound board. Often these become leaky and can cause intermittent problems before they total short.
Speakers blown: yes this happens more often than you might think. If the game was in a noisey arcade, the volume could be up so loud it blows the speakers. You can test the speakers (with the game off) using a 9 volt battery. Momentarily hook the battery up to the leads of the speaker. You will hear the speaker cone pull in if the speaker is good, when you attach the battery to the speaker. Make sure you check the speaker in the bottom of the cabinet too. Often if one speaker is blown, the others will not work.
Main amplifier is bad: the sound board uses a LM1875 as the main amplifier. This device has a large heat sink attached to it. Often, this component has heat failure. The sound works fine until the game warms up for five minutes or so. Then the sound starts cutting in and out. You can use a logic probe on the leads of the LM1875. If the probe's beeps correspond to the cut in sound on one of the leads, the LM1875 is probably bad.
If the LM1875 isn't at fault, check both of the op-amps too. Depending on the revision of the sound board (DCS or pre-DCS), these audio amps can effect a certain type of sound they amplify.
On DCS games, the DAC for the DSP chip dies, and the TDA2030 amps are pretty fragile too.