If you press the self-test switch (the one ON the S&T board) during game play (ie, after that "crackle"), does it play through the sounds and speech without issue? If the S&T self-test works consistently, check your cabling to the board. If you've not re-flowed the S&T connectors, check them for cold solder joints.
If the S&T self test *always* works, but doesn't want to work during game play, it's potentially related to a flaky connection to the S&T, but could be the U11 PIA on the MPU board flaking out once it heats up as well.
You can try swapping U10 and U11 PIA's real quick to see if that makes a difference, or simply swap out the U11 PIA with a known-good one to confirm. Verify the U11 socket looks good while you're at it.
If the S&T works when you first power it up, but then acts up once it warms up a bit, you've got something else going on. Check the incoming voltages (12 volts unregulated, and 6.3 volts), as well as the on-board voltages (+5, -5, and +12) to ensure they look good. Verify if you get the full suite of S&T boot flashes (flicker + 5), even when you're not getting good audio - if so, then lean towards the cap kit next.
If it's (the S&T) no longer booting fully once it warms up, re-seat socketed chips, test again, and then use the flash count as a guide on what to look at next.