Quoted from 72Devilz:
I use a higher end unit now for work but I used to have one like this and it is affordable and it served me well. amazon.com link »
That's cool! Might have to add one to my Christmas list.
Go figure, after all this the sound magically started working again.
Of course, now we're having intermittent problems with the playfield switch connector on the MPU. I replaced the headers and female connectors but this board is very fussy about accepting new solder. I'm hoping someone comes along with a replacement MPU, but I'll see what I can do in the meantime.
For now, I have to assume the sound problem is/was a connector issue and I'll treat it as such until further notice. If the problem comes back I'll know what else to try too. I'll post back with any updates.
Thanks again everyone for all the guidance and info!