ok guys for dlc cars it's a bit complicated but I know how to do.
go to : mods/update/x64/dlcpacks/(choose the dlc folder you want)
exemple : mpimportexport/dlc.rpf/x64/audio/sfx/dlc_importexport
now, choose the the car file .awc you want.
exemple: italigtb.awc , right click then and export to open format.
now: in italigtb folder you are extract search: VEHICLE_EXTRAS_HI_EXHAUST_POP_01,02,03,04 and 05. is the backfire sound.
now take the version of the mod you want (version 1,2 or old) and rename your files exemple : 0x0AC37C55 to VEHICLE_EXTRAS_HI_EXHAUST_POP_01 and replace it. even if there is only 5 files with name VEHICLE_EXTRAS_HI_EXHAUST_POP, with the version 1 of the mod i have 13 files. so take the 13 files and change it by VEHICLE_EXTRAS_HI_EXHAUST_POP_01,02,03,04,05,06,07,08,09,10,11,12,13.
after just replace italigtb.awc with your new. ATTENTION, EACH CAR ARE A DIFFERENT NAME FOR POP FILES.
now for your ADDON CAR
just go to your vehicules.meta to your addon car and search the line <audioNameHash>
exemple <audioNameHash>ELEGY<audioNameHash> and change ELEGY with the other sound car you want.