Dirty rom legs? Bad rom socket? Bad rom? See if things change when you reseat the rom chip.

Reseat the bottom left side driver board connector. This feeds to the sound board to activate the different sounds.

The header pins on almost every WMS System 3-7 sound board I've seen have had cracked solder joints. And, the original pins were the craptastic round pins. It couldn't hurt to replace those headers.

And, perhaps obvious, but I always recap those boards.
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Does the static come after the normal game over sound or in place of it?

If the static sound gradually disappears it would seem to be a heat related issue.
I would go with Chris' suggestion and change the male connectors on the sound board to rule out solder cracks or cold solder joints. As a rule I change these as a matter of course.

I think we need more info. Is it erroneous digital noise, like is the sound board playing random sound calls when it should not be? Or is it amplifying hum/static from ground loop/lamp switching?

When i built a test fixture I noticed the routing of the volume control pot wiring can introduce noise. Just holding the wire near the display board picks up the display switching. The lamp matrix can do something similar.

I am almost going to rig up volume pots with real short wiring harness and leave it in the head near the board for games with sound boards that pick up a bunch of lamp and display switching noise.

The two by three square plug on the power supply with black carries the and lamp and sol ground from the rectifiers and lamp filter cap. That seems to be a spot of ground loop hum comes from. I noticed just touching that plug on a laserball removed a bunch of hum. The plug was burnt.

By curiosity, I would test with speech board unplugged.

It's also skipping in the middle of sounds. Does that go away with time too?

The sound when the game is over is choppy.

That sounds like digital junk noise.

When the game ends and it is playing odd noise, logic probe the PIA pins 2-9. If you get pulsing activity the PIA is telling the DAC to make noise. I think it would be more likely to come from the sound effect DAC than the speech chip. Basically you want to try and figure out where that noise is coming from.

The software doesn't seem to do any memory testing and i've seen some odd crap like that when the RAM or PIA is bad. My best guess. Too bad those chips are usually hard soldered to the board.

That's horrible. Are all the grounds good? Ground pin not broken off of power cord? Good grounding in your house wiring? Did problem start after LED's were installed? Anything else not factory?

Couldn't hurt to try floating the sound boards by using plastic screws and washers, to insulate it from cabinet ground.

While it is very important to have a proper intact cord and a grounded outlet, the lack of ground on the primary side of the transformer is never going to be the cause of machine issues. The ground on a power cord and house wiring is in place only for safety. The current on the hot wire must be equal to the current on the neutral with zero current flowing to the ground prong. Current is only carried through the outlet ground wire if/when something goes wrong, is mis-wired, has a short, etc. If this ground was ever used as the normal means of carrying current, you'd always trip a GFCI due to the resulting imbalance in the hot and neutral wires.

Replace Sound rom 3
No?

Replace the RAM 6810

Speech board removed Jumper:

A jumper is placed at W1 for sound to run without speech:

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At the start of the second game, as it says 'Firepower' the junky noise is still running, then immediately quits when it's finished saying 'Firepower' & starts the first sound effect... if that points to anything.

I noticed that too. I think that points to the noise coming from the sound card l, not the voice card.

You can test this by turning the voice/balance pot to one extreme, then the other when the noise is playing. My bet is it’s coming from the sound board part.

Did you try swapping the cpu chip between the sound board and the mpu board?

Just for kicks, what happens when you push the sound card diagnostics button?

I’d try a different rom in the sound card.

Interesting. That is PIA ports PA7 and PA8 which goes to the most significant bits inputs of the DAC. I think that points to the PIA or DAC if touching the probe there makes it stop.

I wonder if the PIA has a blown output and the probe is dragging the signal down or up? Does PIA pin 8 and 9 show a valid signal high or low? Does it seem like the PIA is trying to drive those signals? If you push the test button on the sound board all of those pins should be pulsing. Take note if any only blink the high light, only blink the low light. never changes state or give no reading.

Is the PIA made by... arghhh... AMI ? If so i'd be quick to suspect it. So many issues with AMI parts.

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Suspect a Corrupt 6810

The PIA is doing what it's programmed to do, the system never crashes and can repeat the same problem over and over points to a bad/failing bit on the 6810

Garbage coming from the RAM to the PIA during the game over sound instruction.

The PIA shares the same signal that has garbage coming out for other sound selects however no other sounds are having the noise.

Cut the PIA pin 9 trace and check the behavior

Re check hi low port behavior against other 7 ports

Ok now? Replace IC 13

Difference continues?
Replace IC 10