After playing around with the latest dev-branch (126674) of Arma 3 I realised, that the engine sound of all vehicles are too quiet (in some vehicles nearly not existent), if you get in, start the engine and fly or drive around.

You can clearly hear all outside environment sounds like crickets, birds and the ocean, but you can barely hear the own engine sound.

If you get out of the vehicle, all sounds are normal.