are you getting bored of listening to the same engine every time you start the game heres you chance to vote!
This is planned. I've been told that the Covet was a bit experimental, but improvements to that and the addition of unique and realistic sounds for the rest of the engines will be coming.
The birds really need work. I mean I never hear them! I would just LOVE to wander around East Coast hearing birds everywhere.
*I'm and on the topic at hand, here's a great soundpack that gives each car a unique and realistic engine sound http://www.beamng.com/resources/engine-sound-replacements.63/