I have found that the engine sounds are in .flac format. I can't play them, because this error shows up when i try to open them. I also can't extract or copy them, because the extracting process doesn't let me proceed with these .flac files.
The engine sound files are encrypted because they were purchased from a third party by the BeamNG team. Why do you want to unpack them anyway?
That explains a lot. I just wanted to know what different sounds they use to make the engine sounds (valvetrain, gears, bearings etc)