Positive progress
I remembered that I had gotten a Bally/Midway SI-II sound board in a lot of misc boards off eBay a few years back. Found it and it had a SN76477 chip soldered in.
I dug out my desoldering station and 56 stubborn 35 year old solder pins later, I had both chips free. Installed 2 sockets and then moved the B/M one over to the Taito.
Fired the game up, went into test mode and TADA... UFO Saucer sound working
Now, to continue to attack the clcking sound. I noticed that whoever had it before me had socketed the LM377 and pin 9 was floating and in fact when I pulled the chip, the socket leg pulled with it. I put it back in and there was nothing to solder it to as they had lifted the pad on the bottom. I checked the schem's and it showed it was crossed over to pin 6. So on the bottom, used some 22 gauge wire and crossed it over pin 9 to pad 6.
Put it back in and suddenly the game was twice as loud! I had to readjust the bias of each sound to not overtake each other as much, and tweak the master volume. At least this now lets the background thumping to be louder that was really quiet before.
Finally, annoyed that the clicking sound only happens when the base is destroyed in gameplay but not test mode, I fired up a game, but accidentially hit 2 players. Interestingly enough it did NOT make the sound when I went between player 1 and 2 dying. However after the last player 2 death, it DID click then after player 1 depth (I had got a free base there so played longer on that player) it clicked! It also clicks once after initial boot.
Therefore clearly it is not something specific to the caps/sound board -- some instruction is causing the sound to go through. All other sounds are fine. Since it is related to player death/game redraw, or final one, it must be some mode the CPU puts the board in to power off and back up the sound chip or somehow affecting the voltage to it and therefore may be more of a power supply issue instead of sound board issue.
That's all the info I have now.. I don't know if I will spend any more time debugging the click--everything else works. If anyone has any more info to share on that, it would be great to help me and the other person here with the same problem of clicking.