Right now if someone flies fast over me with a jet I don't hear a thing. In real life, you can hear planes from an incredible distance away. I would love to both have sonic booms heard by other players when a vehicle is traveling past the speed of sound, and in general longer ranged sound to be more in line with what you might realistically hear. It would really add to immersion and the cool-factor of someone flying a craft rapidly overhead.
(Of course none of this would apply in space because it's a vacuum).
