Found this on the HTC site: I just figured out the call volume problem on my HTC One. Under "Call" on the Settings menu, there is a "Hearing aids" option that was selected (by check box.) When I de-selected this option, the over-amplification of incoming conversations resolved. The first clue was an ear-shaped icon at the top of the screen with the service bars, 4G and Wi-Fi icons, etc.
This fixed a long-standing problem for me that T-Mobile support couldn't figure out, either.