Please keep it simple to match my understanding of these, I'm mainly an EM guy.
After playing fine, mid session, wouldn't kick out a ball at start of game. On test, only flippers, gate, and coin solenoids fired. Good voltage on playfield at non-firing solenoids, grounding transistors activated them. Good readings on test points on SDB.
I recalled reading somewhere that the sound board could interfere with solenoids, so unplugged that, no change, but after plugging it back up, the solenoids are working. Does this mean just a bad connection? Any further diagnostics I should do? Should mention that the sound board doesn't play the correct tunes, but somewhat random in a minor key. The sound has changed from when I first got the game.
Thanks for any suggestions.

I sent the MPU out for an overhaul when I first got the game and replaced the connectors to it, so will assume the trouble is at the sound board. Not sure my soldering skills are fine enough to reflow the header pins, but could give it a try. What is a good replacement sound board?

Thanks, guys, for the info. As time permits I'll check this and report back.