What is the value for the lamp matrix fuse and is it slo blow or fast? The lamp matrix is pulsed, so instantaneous levels of current can easily blow an incorrectly rated fuse.

Sound card rework is probably unrelated to the lamp fuse issue. Just because the capacitors were replaced doesn't necessarily mean the root cause of the fading sound was addressed. It could be something else causing that problem.

That would be the correct value for the modification. Were you having any problems/symptoms of trouble with the lamp matrix before adding the AC side bridge rectifier fuse? (or don't you know because the game came with it already installed) The power supply also has a fuse for the lamp matrix DC side - it is F3 - this should be an 8A fast blow. Is it correct and in good condition?

As far as the sound problem, based on previously replacing all the capacitors, I highly doubt those are the problem. It could be heat related if the game sounds start off fine and then drop off as the game warms up. Especially if you can repeat this by shutting off the game, letting it cool and then get the same symptom when you turn it on. You might be able to pinpoint this with a can of freeze spray. A likely culprit could be the final amplifier IC1, or possibly IC13

Yes. Electronics freeze spray is safe to use on energized circuits. Best if you have some audio running during testing. Pinpoint spray a specific component for a few seconds and listen for changes. You can use your sense of touch as well. Some components naturally dissipate heat, but if you leave the game running a bit and touch (carefully) you might find something running abnormally hot.

The same type of symptom as this can be mimicked by capacitor problems (shutting the game off allows capacitors to discharge) but you already replaced those so this is somewhat less likely. Good luck, hope you find something)