@0rganiX Hello! I'm Hallel and I'm gonna be helping you answer your question.

First of all: Are we reverting replacements to addons or addons to replacements?

Step one: Open the Winrar Archive
Step two: Go to File Explorer then find your desktop folder
Step three: Create a folder and name it sirens
Step four: Go back to your Winrar archive and drag the vehicles folder and the vehicles.oac file into your sirens folder.
Step 5: Open OpenIV (You MUST have) Here's the download link if you haven't yet installed it: https://openiv.com/
Step 6: Go into Edit Mode.
Step 7: Go to path: mods/x64/audio/sfx/resident.rpf
Step 8: Click new on the top.
Step 9: Click Import Open Formats
Step 10: Locate your vehicles.oac file and double click it.
And you are done! Double click the vehicles.awc file that's in resident.rpf then find SIREN_PA20A_WAIL then double click it and listen to the siren and see if you did it right. If it sounds the way you want it to sound like, chances are you did it right.
And that's it! Enjoy your newly installed sirens!