I've got a cockpit, it has 4 volume controls behind the coin door. The problem is the sound for passing cars is MUCH louder than everything else, especially on the left side which has blown the left speaker.
I found the answer to my question though, on the sound board:
vr1 - skid
vr2 - alarm
vr3 - crash (large)
vr4 - crash (small)
vr5 - ambulance
It was printed on a sheet inside the back door. I can turn them all up to be closer to the passing car sound, but there's a bigger problem - the left side pass sound is way too loud even with the left volume control minimum. Swapped the speaker connectors around and the problem moved with it (but the distortion stopped), so the left speaker is definitely blown and the left channel is making some sounds too loud.
I think I'll get a spare sound board and compare, could be that or maybe one of the amp units.
Also found the big 12" speaker has no foam anymore... Cone held in place with a scaffold of cable ties.