it's so odd that they made different T unit sound boards for the different games, there must have been a reason, there's a definite difference in the power harness i know that. how does the bottom of the soundboard look? has it been repaired before? are there any deep scratches (obvious trace damage)?
I think u mentioned earlier testing the voltage at the sound board molex and it was good on the 5v line correct? silly as it might sound but check all the chips on the sound board and make sure all the legs are socketed.
You might as well look at the bottom of the main PCB also for repairs/scratches as they could cause problems also. use some super fine grit sand paper and sand the legs of really dirty/corroded chips till they shine (don't sand a lot just enough to make a better contact)as mecha said. do this carefully as the pins are easily bent/broken and when you remove them from whichever board you are working on, i'd start with the sound board as you can't make it any worse than it is really. Make sure the chips are facing the right direction before and definitely after you pull each one (powering up backwards is BAAAAAAD). Since the game plays i wouldn't mess with to many of the main board proms i'd try and concentrate on the ones that control the sounds only (make sure you check the socket leg solder joints).
Just some more ideas as i read over this post.
Also, do you have another boardset (any jamma game) that you can throw in there to make sure it's not cab wiring or your speakers?