I had 2 system 80B mpus go down just recently in good working games same sorta thing just plain dead. Both quit while sitting in attract mode. One was a bad U4 RIOT the other was a bad U6 RIOT I already had them socketed to they were easy to test and swap out. Could also be bad connections on the daughter card. If they are suspect I typically just buy new daughter cards from Fred since the originals are easy to damage with only having single sided traces.
Sound issues depend on whats wrong. Are you missing all sound just some sounds or are they just distorted and glitchy? The most common thing I have found is bad op amps on these sound boards. Rams are not socketed so they are not easy to test. I doubt it would be a bad rom but anything is possible.

Problem is there isn't really a quick simple one size fits all method to fixing these 80B machines. All seem to have the same issues and there is really no better method than to go through the whole machine all the boards and all connections little by little. You seem to be on the right track.