Due to a suggestion of @TobimanW, I made this little siren modification.
You can use it as a standalone or as a plugin for my siren pack, but then my siren pack has to be installed first.
Sirens contained in the RTK7 Siren Mod:
RTK7 Land
RTK7 Stadt
YELP

My siren pack: https://www.gta5-mods.com/vehicles/gta-v-german-sirens

Installation:
1. Go to mods\x64\audio\sfx\RESIDENT.rpf\
2. Right-click vehicles.awc and choose "export to Open Formats (.OFT)."
3. In your new vehicles folder you have to drag and drop the files and replace the old ones.
4. Now click on new and choose "import Open Formats file."
5. Choose your vehicles.oac. When it has been imported you're done!

Feel free to express your criticism or a rating!
Thank you! :D