Well, I replaced the 74174 and that didn't help. Took the boardset to the game along with my logic probe, and it seemed that I was getting regular pulsing off of pin 10 of E4. Just for kicks I pulled the LM3900 at K4 to make the sound go away - and it didn't! WTF? So, I used that LM3900 top piggyback some of the other LM3900's, and when I got to the one at M4, the sound decreased quite a bit - but was still barely audible. As M4 was socketed, I pulled it and the sound went away, even with the K4 installed. I replaced the LM3900 at M4 and the sound is now working 100% properly.
Strange, as my schematics show M4 to be for the MISSILE SOUND, and that sound was clearly working normally when I played the game. Just have to replace the backlight (which I pulled for another game) and it's done.
Oh well, time to move on to a few other games that are acting up:
Robotron - I need to install the 4164 (they better all work as I barely have enough)
Lethal Enforcers - graphics glitches in game play
Q*Bert - plays the wrong sounds in certain spots
Tempest - turn down brightness
Gyruss - install interior cardboard bezel
Hopefully I can get everything working in time for Thanksgiving. if not, I can pull some from the garage to replace them with if they don't want to be reasonable...