Since the incident with ROBLOX and the audio lawsuit etc of course a lot of siren sounds were taken off ROBLOX, meaning an emergency update was added to just give every emergency vehicle the FSP siren set for the time-being.
I personally think it would be cool for some of the sounds for various sirens to be changed up a little bit to allow for some variation and add some realism.
You could give City PDs a rumbler siren too.