sorry late reply, i haven't done this but...
assuming your using window's explorer:
open an explorer window #1:
go to the car directory that you want the sound upgraded ../content/cars/hate_the_sound_car
copy the "sfx" directory
paste ( this will create a copy in the directory, named something like : "copy_sfx" directory
open another explorer window #2:
goto the car directory that you like the sound from ../content/cars/LOVE_the_sound_car
copy the "sfx" directory
go back to explorer window #1
paste (replacing the crappy sound "sfx" directory )
try and enjoy! (remember if it goes pear shaped, you have the "copy of the original" to fall back on)
if you want DOS command line, yell out.