Not at all.
Low really isn't all that noticeable, and on all of mine nothing really happens on medium until ~45mph.
You will not see a change in the volume setting number on the dial, that's the base reference point.
I once thought I heard it doing something, now I have to admit I sense no audible difference. At the same time, I don't think a car is a very good environment to discern small audio level changes, so I can't say it's not working, just that I don't hear a difference.