SenatorB

Glad you've made one yourself however just so you know that is the RSG MCS32, which is the most common used police siren around United Kingdom and are most popular to be known in londonn and Obviously Birmingham like you qutoed
As the Metropolitan Police quoted, 
 
 
If you would like to know some more information about the vehicles here is the link to the page
https://www.met.police.uk/SysSiteAssets/foi-media/metropolitan-police/disclosure_2019/august_2019/information-rights-unit---sirens-on-police-vehicles

Just in case you still haven't made your mind up yet, I've composed a list for you. Enjoy! ;)
 
LSPD PACKS:
 
 
 
Some Sherriff packs for you too, these include Highway Patrol and State Patrol:
 
 
 
 
ALSO; this is a pack I recommend you to check out although they are liveries, I highly recommend for the nostalgic feel for all the agencies in the game:
 
 

I've found with a lot of FiveM servers you get some of the following:
SIREN_PA20A_WAIL - Police Wail 
AIRHORN_EQD - Police Bullhorn/Airhorn
(Yelp and Priority Tones are typically stock)
SIREN_FIRETRUCK_WAIL_01 - Firetruck Wail
SIREN_FIRETRUCK_QUICK_01 - Firetruck Secondary
 
I'm not fully sure how I got these extra tones, but some FiveM setups use these so I figured I'd post it. Be sure to install stock versions and these as servers can vary.

Any mods that have this option? instead of being #1 to get to a call, have another unit or two respond as well and sometimes they get there faster?

I was installing some sirens today and I noticed the same thing, air horn went from the hash hex number to what I believe now is called "AIRHORN_EQD.wav" and the wail became "SIREN_PA20A_WAIL.wav" as WanderEDG said. I'm thinking OpenIV got updated and they updated the siren hashes names too.

I ran into the same exact issue. I believe it's due to the fact that OpenIV was updated last week and some of the audio files in the vehicles.awc have been renamed from the hex number they used to be (0xSOMETHING.wav) to actual names. I found that the main siren is now SIREN_PA20A_WAIL, the airhorn became AIRHORN_EQD and the rest are still SIREN_2, AMBULANCE_WARNING and POLICE_WARNING. What you should try is get the vehicles.awc file, extract it to open formats to your desktop and then take your custom siren and before dropping it in there, rename 0x0D329446.wav to SIREN_PA20A_WAIL.wav and 0x0EA58C7C.wav to AIRHORN_EQD.wav and then make sure that when you drop them into the vehicles folder you get a prompt to replace all of the ones you're dragging (usually it's the 5 I listed).

Well, for users like me, it won't work until script hook v update is realesed. 
I've installed too many mods. Can't uninstall them "quickly", because the epic games launcher doesn't support "search a game" or wathever it's called. So you can't use a backups, epic games launcher won't run the game after that.
 
God damn rockstar north🤬