5.07 isn't very high, particularly not for this game (you have to consider you're powering 3 boards: the main board, the expansion board, and the sound board).
your test point matters too, I can't remember exact numbers, but I had a drop in voltage from the JAMMA edge to the sound board. in the grand scheme of things, I wanna say I ran 5.15 on mine, you can go higher than 5.00, just not TOO high. once you start approaching 5.30, then it's risky, unless it's a game that consumes a lot of power.
for what it matters, Randy Fromm uses 5.10... I personally go a little higher than that, 5.15 at the JAMMA edge, that's a pretty solid voltage. I always tell the story about our Road Burners on this topic: this is a game that mind you is hard drive based, running 5 and 12 volts, and the +5 at the power supply was 4.86V. I only noticed the problem when one day it errored and reset with the "3 strikes" read error from the hard drive. Fromm's Blue Book cites a story about a WWF WrestleMania (a Midway Wolf unit, like MK3) that would reset when the voltage was at 5.00 or greater, but ran perfectly fine at 4.9V. each board has its own voltage tolerance, I guess is the moral of the story.
your ideal test point would be an IC on the board, but I honestly can't remember off the top of my head which legs you test from (mod, someone smart, help me here

) and there's a high risk that you can "slip" off one of the legs and short them together, so I only advise this once you're pretty handy with this stuff.
in the case of MK2 though, I'd say the safe test point would be the sound board, not sure if that's in fact where you checked though. it's worth noting that on switching power supplies, only the +5V is "regulated" (meaning, that it will run only at the setting you made at the adjustment pot) and all the other voltage rails will be a little imperfect. my MK2 metered at around 13.2V on the +12V, it's nothing to worry about.
considering my Road Burners story, and how the game ran perfectly fine for over a year in a business setting, I'm inclined as to say the < 12V that you're getting is not the cause for the lack of sound.
I'd lean towards checking the roms (as I mentioned in my previous post) that they're all seated in, that your cabling is sound (run continuity checks on each wire from the main board and sound board headers that it plugs into), and inspect all the boards for broken traces. the Williams/Midway stuff is pretty easy to check this for, it's nothing overly sophisticated and advanced and fine-traced like the Japanese manufacturers.
also, fun little tip for your sound ribbon cable -- if you don't have another one to test with, you can use a regular computer IDE cable instead. just make sure you have the orientation of the cable right (red lines = pin 1). you can use a "keyed" IDE cable (with one of the middle holes blocked off), one side of that keyed hole will fit exactly on the ribbon cable connectors on these boards.
add that IDE cable check to your list of things to try. otherwise, there was a big name user on here that recently had their MK2 up for sale *wink wink*
good luck, sorry for the mouthful, but I enjoy passing on my useless experiences to others so they avoid the hassles I went through.