This idea in mind would be a great addition for vehicles, basicly it's a support add-on much like the interactive vehicles and custom vehicles support add-ons that it adds sounds to a car that it has a .cs file to be identified by the script, there could be a line on the script that allows you to choose which type of engine sound to fit the car type, and it could include crash and start up sounds (maybe sirens and horn) The add-on could include different engine sounds or the vehicle itself could include a folder inside with it's own custom engine sounds that runs on the support add-on, there could also be a RTB pref to turn it off.
I know it sounds complicatd but if Port somehow makes his smooth transition engine sounds to work with this it would be a fantastic thing for vehicles, sorry if I coudn't explain it so well.