I notice that the explosion sounds don't work in the main arena, but do in the SEA and offline...guess it's off. Only time I hear the nifty explsion is when I'm back in the tower after dying, or when I hit quit.
(Btw why is there so much HD activity after long vehicle missions?? I've seen it sit there for 1 min or longer before it goes back to the tower

)
But I've read where mostly pilots couldn't hear anything beyond there engine. Not even machine gun fire

and they felt incoming fire when it hit the plane, where we have the hit sound to compensate (well mostly)
But I'd like to think you could at least hear other planes, ack, vehicles etc from the ground or when at low throttle in a plane

- Jig