Quoted from PinRetail:

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!

Quoted from PinRetail:

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.

Quoted from PinRetail:

This is a tough problem to give you advice on.
From your 'Sidetracked Brewery' line in your profile, I think you might be trying to keep a machine made in 1994 working in front of the public. I've always thought that this is a little like trying to run a taxi service with twenty nine year old cars. (Cargument...)
If this machine is in front of the public, it's important to get it running quickly, and getting unreliable parts replaced with reliable parts might be a very good idea.
Yes, $400 for a pinsound board is enough to make me choke... but it would get your machine working and you wouldn't look back. You could also sell your old board working or non-working to get back some of your repair cost.
If you've got weeks/months to fuss with this, you have a couple of options.
I'm not afraid to pick my battles. If a board can be fixed easily, I'll do the simple stuff, maybe dig a little deeper. But fairly quickly in the board repair cycle I come to a choice...
I can send this for professional repair. They have the parts, they've done this before.
I can attempt to repair this myself, but at my level of skill, it would be extremely helpful to have a known good, working board on my bench to set right beside the non working board. Testing things becomes easier, and I can swap suspect parts into a known good system. (This solves your PAL question...)
So, do you have access to another working machine with the same sound board? Can you purchase a working sound board that you can use as an example for this repair and then sell the working soundboard you just purchased back?
The hardest path is to not have a working example of the board to test with/against, and to attempt the board repair without access to all the parts. That's likely going to be a... memorable... repair. Hehe.
I'm in Charlotte, and I'm sorry to say that I don't currently have inventory of the sound board you need... otherwise, I'd have been glad to help you by either providing you a known good machine to test your board in, or by sending you my (working) board to give you something to test with. Are there any other pinball guys near you that might provide some help?
If you proceed with your board repair, keep posting what you find, and we'll hopefully get the attention of some of the better board repair guys on Pinside involved.
Good luck!