No music in test mode, but you have speaker amplification of the CPU generated sound fx? If so, your FM synthesis chip output is the issue.

It could be a few things. Might not be enabling, could be a faulty summing opAmp (U16), faulty Yamaha FM chip(s) could be bad (U9, U10, not likely though), etc. Try re-seating the FM chips first. Suspect something very simple here first. Check voltages on the audio board. Make sure it has +5v logic power. If that doesn't help...

The FM output comes off of U16, pins 1 & 7 for the two channels. You can probe those pins for DC voltage of any kind while in music test mode. You can also connect an old junk pair of headphones or even a piezo disc with leads coming off it after the DC coupling caps (c5 & c6) to listen for a very faint FM output. If you see/hear nothing, probe the non inverting inputs on that opamp (pins 3 & 5) and check for voltage in test mode. You're just looking for anything wiggling around that would indicate audio playback. You can audio probe there too, but with no coupling cap, you'll be applying DC to that speaker source (so don't use something you care about). You should also see +12v on Pin 8, and -12v on pin 4 of that 1458 opAmp.

If you see nothing there, I have some other ideas for you to try.

You should replace the electrolytics on these, minimally the larger value ones for the audio amplifier ICs. I think people often overlook the audio boards, but they need power filtering too. Are any of the caps vented or puffy looking?

i've never seen one of these yamaha chips fail, but it isn't impossible obviously from that thread. Have you probed the soundboard cpu or pia bus for activity?

I'm going to guess you have a damaged trace on the bad MPU there going from the data/address bus on the sound cpu/pia to that connector. Luckily you had another board to test with there. Makes it easier to debug now. If your sound 6802/8 CPU on the bad MPU was faulty, you'd probably not get partial operation. It would just not work. You have voice, so your CSVD and D/A section is good too.

I'd do continuity checks from the sound CPU & PIA to the connector pins on the bad MPU. If not a trace issue, the PIA is probably suspect.