Turns out for me it was a mod problem. Sorry about that. Seems fine now. I agree that there could be frequency issues with people not hearing certain frequencies.
Yellow belts also seem a bit louder to me than red and yellow because the sound deeper. Faster speeds have a more high frequency whine and that decreases the volume for me. Turning yellow down to 70% and red to 90% seems a lot more consistent
Electric mining drills and assemblers are a bit too loud IMO. I turned those down to 60% and 70% respectively. Not sure about the "right" AM value though