Since it is the Ram chip for the game that stores the volume setting, you have a 'settings not held' problem.
So. Replace the batteries. Examine the battery contacts, make sure there isn't anything tarnished, corroded, and that the pads aren't falling apart. Pro tip: Replace the batteries with the power on and you won't lose any high scores or settings.
Measure the voltage at D2 according to this section of the pinwiki:
Personally, every chance I get I'm replacing the ram chips with NVram replacements.
This requires installing a socket, which is not an easy thing to do if you haven't done this kind of work before.
If the settings ram is getting corrupted somehow, either the battery isn't supplying good power to hold the settings, or the ram chip itself is the problem. It's better to get rid of the batteries entirely by installing NVram, and that also replaces a potentially bad ram chip.
(I'm glad you aren't finding this to be the volume control chip. I've had only two of these fail, and both times they went to full volume that couldn't be turned down. Replacing the chip solved the problem in both cases.)