Of any part of the simulator, sounds are by far the most subjective. Most simmers seem to prefer an aircraft to sound as they experienced it. So, like it sounds in the cabin because few simmers have experienced how things sound from the flightdeck. They also seem to want sounds like flap motors, even if they are not audible from the flight deck in many aircraft.
PMDG is highly reluctant to give in to that pressure. Even though what is delivered does not always meet the customers’ expectations. I have my sound setup like this, as I like to fly as if I wear a noise-canceling headset, like almost all pilots do. This comes closest to my personal experience on actual flight decks wearing a Rugged Air RA950.
I am not a fan of the built-in headphone simulation.