I’ve never seen this on a SAM game. So it will probably be something odd. My first thought was going to be something leaning on the volume button. But you saying the value is normal tosses all that stuff out.

So if it was me, the first thing I’d do is a full reload of the game code. I’d use a copy I freshly downloaded from Stern to make sure nobody added any pinball browser stuff that had bad levels. Then a full factory reset with the new code. If that didn’t do it, I’d look for bad caps (the ones in the circuit of the back speakers).

If that didn’t handle it (sorry, I have a few games), I’d swap out the MPU with another game to check if the amp circuitry had an issue.

All of this is just trying to eliminate possibilities until you find it.

i had the same problem on two different SAM spiderman a long time ago.
I remember that it was a bug in the code, that happened only when the setting 41 in the standard setting menu, "Music volume" was changed.
The factory value is 1. I reset it to 1, and had never had the problem anymore.