Extract "GTA V Other Backfire Sound by Hippy.zip" anywhere you want.
- In OpenIV, Select edit mode and click "Ok", and go to your GTA V folder, then go to x64audiosfxRESIDENT.rpf (or modsx64audiosfxRESIDENT.rpf if you use that).
- Right click on "vehicles.awc" and click on "Export to opnFormat (.oac)" (Extract it wherever you want). It will create a "vehicles.oac" file and a "vehicles" folder.
- Now go in the Other Backfire Sound mod files you extracted, copy all the files of the "Files to leave empty like that" and "Modified Backfires" folders, and put them in the "vehicles" folder that got created when you extracted "vehicles.awc".
- Go on OpenIV, and again, in x64/audio/sfx/RESIDENT.rpf (or mods/x64/audio/sfx/RESIDENT.rpf if you use that).
- In OpenIV, click on "New" (top left) then "Import openFormats".
- Choose the "vehicle.oac" file that got created when you extracted "vehicles.awc", and click Open.