nobody is going to make a mod to port the old sounds to the latest version of beam. they are locked away in the old game files via a password, as beamng does not have rights to the sounds, they are paying royalties to use some of the audio in the game. install the old version of the game (0.14) if you want them, but i don't know why you would want to do that. the starter sounds are generic and the audio system as a whole has improved so much since then.
yeah, its because 0.4.0.6 is the last version to support dx9, while 0.14 is the last version to support dx10. dx11 support is ongoing. good luck getting them