red sauce

Reinstall it.

In the vehicles.meta you could change the audionamehash to police or any slot that uses the siren you installed.

Ok.

In OpenIV go to mods>x64>audio>sfx>RESIDENT.rpf and click on vehicles.awc then right click it and choose export to openformats. choose a folder/make one and select it. You should now have a vehicles folder and a vehicles.oac file. We come back to this later so leave it open but minimised.
 
Find the location of the siren you downloaded, I assume it is set for police sirens? If so its names would be all or some of the following: 
0x0D329446 - Police Primary SIREN_2 - Police Secondary POLICE_WARNING 0x0EA58C7C - Horn You need to rename the siren audio files to the names of the fbi and sheriff2 (granger) sirens.
0x0A8960B6 - FIB Primary 0x0A49B203 - FIB Secondary 0x0E0DA7BC - Granger Primary 0x16E74B3E - Granger Secondary So you might have a audio file called "0x0D329446" right click on it and rename it to be 0x0A8960B6 for FIB primary. Rename "SIREN_2" to "0x0A49B203". The copy these files and paste them into the vehicles folder which you exported from openiv say yes to replace.

Go back to the sirens you downloaded and renamed and repeat for the granger sirens. 0x0A8960B6 to 0x0E0DA7BC, 0x0A49B203 to 0x16E74B3E  and copy replace as above.

BEWARE the vehicles folder can not exceed 15.7MB, if it does vehicle sounds will be muted.

Video for help if needed: https://www.youtube.com/watch?v=0RXLRusbNAs 
(sorry if we are not allowed to post our own videos. I do not care about promoting my channel as I dont even upload to it, just thought this would be a good visual assistance to the user in help. Will remove it if needed)