What are the possible SIREN string options available for the VCF file of ELS?

For example if I want to add the granger, the fbi and policeb siren strings, what are the codes for that? I know for the main police siren is "VEHICLES_HORNS_SIREN_1"

I'm not sure thats entirely possible yet. I've tried my self to guess some of the strings however havent got any luck. All the possible strings are listed in the bottom of the els documentation, believe its page 28. However the answer to your question is that we have to wait for them to find and implement the strings.

I really want to change my siren on some cars on ELS, having all Police cars (and firetruck) sound the same is a tad annoying. Especially using the british siren

I completely agree with you guys that it is annoying and repetitive hearing the same siren on all ELS emergency vehicles in-game. It's just a matter of time until ELS V gets an update with the other sirens' audio strings functioning. Hopefully won't be too long, seen as ELS hasn't had an update in nearly two months.