Make sure the game isn't running before you do this. The change will not take place while game is running.
Just say you wanted to change the sound of the Chargers engine..
Find a car with the sound you think it should have..Say, Eagle MK4.
You open the Eagle MK4's decriptor and find a line that says the car sound eg. "Sportscar2".Copy that line.
Open up Chargers Decriptor and where it says its sound eg. "Car1"
Back up the Charger Descriptor first, then replace "Car1" with "Sportscar2".
Then in the game it should work. You can do this to ANY car in the game. Cool huh? You can give any car a siren too!