aHeadset is a service maintains different volume profiles for different headset status (on /off). It requires no setup. You can adjust the volume as usual, but aHeadset will record the last volume you set in each status. When you pullout or plug in the earphone, the volume will be automatically set back to your last choice.
On some HTC phones and MIUI, aHeadset will conflict with the built-in headset profiling feature and not work well. (Notice: other phones shipped with headset profiling feature may also be problematic)