Ribbon cable problems are common, but I'm sure you re-seated them, plus, I'm pretty sure it'll boot up with a 'everybody dance now' sound even if the ribbon cable is disconnected.
Obviously, you've checked your voltages, and the only thing suspicious is the 11.5 Volts on the 12V line. Because I'm curious, I'd measure AC on that voltage, and make sure that there isn't a couple of AC volts riding on the DC... You won't find anything here, but it's something to test. I'm pretty sure that the +12v won't make the CPU of the sound board give up.
I double check the volume pot and it's wires, because an open there can be a problem, but I've never had an open volume pot on a data east, just a scratchy one!
The 6809 CPU is a failure point, but I usually get lucky when they fail and they get noticeably hot to the touch.
After that, you start getting into actual board repair territory. Reset line, clock, r/w etc. Address and data lines.
There is one test to check if the ram chip has failed on the Wiki:
https://www.pinwiki.com/wiki/index.php/Data_East/Sega#No_Sound_-_5077_Sound_Board_and_Others
Sound board problems always make me frown. The board repair here is frequently not simple.
Let us know what you find!