I did a test at KLAX with the CJ4, FSLTL, parked at the side of a runway: I can hear the engine of other airplanes roaring for taking off but sound level is very low from the cockpit, very loud from outside. So I think that is maybe a configuration with the Cj4 that limit from inside those external sounds, not sure if it is a bug or if it is setup like that on purpose. Maybe you can ask in the CJ4 support thread (I did not find any related question):