I run voltage at just about every game at 5.05V at the chips. you might have switcher instead of AR3, I wound up tapping the reset and -5V lines into a new switcher, leaving the Sense wires unplugged (white-striped red and brown) and I got it pretty reliable this way.
I was getting those weird rom errors on Marble Madness before, reflowing the interconnects for the rom and main boards seemed to fix that. more problems arose afterwards, like me hooking the P2 start lights up wrong and blowing out the input chip, and the amplifier board shorting out and blowing fuses and apparently both FM LM324 chips. (this is why I offloaded the +12/-5 to switcher instead of coming off that board) and lastly, whatever KLOV says about the main boards only needing +5V is completely wrong, the reset line is 10.3/12V (depending if it's AR3 or switcher) so if the amp board or fuses crap out, you lose the entire game.
your missing sound is probably bad LM324s on the old board though. the sound test on MM would just be silent on the FM test, but it would play the POKEY beeps, so that's how I narrowed that down.
the game has been running reliable in an arcade for 1.5 years now.
