Currently the only way to adjust volume is to use the interior sound volume. The aircraft is rather quiet inside when just sitting on the ramp. I was surprised when I sat in it with the engine running, having been used to the noise of piston engines. But the sound deadening on the engine is pretty stellar. It gets A LOT louder in flight, but that's mostly not the engine or prop, but the wind noise. I tried to adjust the levels in the cockpit to match what I heard IRL when flying the aircraft.
The door sound is rather quiet, but I'll look at enhancing the door shutting "clunk". We might be keeping that too low. As for switches, they were recorded with realistic levels. When the engine is going and especially in flight, there's next to zero chance of you hearing any of those, unless you have the ear of an orchestral conductor.
As for headset master caution - they really are very loud. I'll see if we can add a control, like over the intercom, to control headset volume. There is a control there right now, but it controls the interior volume in X-Plane, not just the headset.