How hard is it to create engine audio for BeamNG? If its possible and not too terribly complex, I'd love to make some custom sounds for this game. Is there a tutorial or any thing like that?
As long as you have audio samples, it isn't too bad. You basically just stick your audio samples in a file, and write a .json file that points the game to them. There's a basic guide on the wiki. The problem is, you need a lot of samples (~10 samples, half on load half off load) for it to sound any good, and you preferably want separate intake and exhaust samples too.
not particularly, I believe the game prefers .flac or .wav files, which any audio editor should have no problem outputting.