If turning down 1 sound would adjust all the other sounds to the same level, we'd be right back at square one. Some sounds are too loud; some too soft.
I myself do turn down my engine, now I don't shut it off, i just turn it down.. the constant hum of the engine gets rather annoying when trying to listen to vox.
Now is this something that should be addressed by HTC? Absolutely not. I think we have more important issues than someone turning their sound down that needs to be addressed. Maps, planes, actual bugs, etc etc etc.
This by no means is an advantage.
But since you asked the beginning questions FX, let me ask you this one..
what in sam h*ll brought this up? lol