In the later "Leo" builds of HTC Sense (version 2.5) found on the HTC HD2, there are some neat advanced ringer functionality available under the Settings tab.
One of those is called "Be Polite" and in essence uses the built in accelerometer to detect when you are moving the phone to answer it. In turn, since you are already on the way to answer or ignore the call, the device can significantly lower the ringer for you--hence the "Be Polite" part.
(By the way, Microsoft? You may want to start doing those sorts of things too--they are after all what makes smartphones "smart".)
Anywho, many of us don't like using Sense 2.5 because it was not build for the Touch Pro 2/Diamond 2, therefore does not do landscape and lags on our wee processors. However, Apristel has found that a lot of the Sense 2.1 builds in fact have "Be Polite" built in, but not activated. Installing his cab will un-hide this feature for you to use.
Now the catch: On some ROMs this works and on others it does nothing. So your mileage may vary. Good luck!