BigNonop

Before the actual tutorial even starts, I would like to give credit to @TheHC09.
Without his effort of spending hours finding the sound strings, me and the rest of the modding community would not to be able to use the sound strings to create ELS multi-siren configurations.
As we all know, ELS only allows 5 tones to be played under default configuration.
But if you dig deep enough, you would soon found out that you can have your own configurations and let different department has their own siren.
This time, my demo video shows the possibility of having ELS to be configured to utilize the 14 different tone slots, which includes:
So how can we actually do that?

First, ELS has to be installed correctly and your vehicles should have their own working ELS xml files.
 
Second, prepare a vehicles.oac install folder with its size reduced.
Personally, I would use the one below as its package size is reduced significantly such that it is possible to install loads of siren into the vehicles.oac without exceeding the file size limit.
 
Next, decide what siren you want and how it should be played using ELS.
For my case, I have:
4 tones from the Fed Sig TM-4 for police, sheriff and paramedic vehicles 3 tones from the Whelen CHP patrol car siren for highway patrol cars 1 Powercall tone, 1 WOLO 846 airhorn tone and 1 tone from the Fed Sig PA 1 for firetrucks 2 tones from the Whelen CHP patrol bike siren for highway patrol bikes (shares the CHP airhorn) 2 tones from the Whelen CenCom Gold for police/ sheriff bikes (shares the CHP airhorn)  
After that, you have to organize the setup and choose which tones replaces specific tone slot.
You would need to name the each sirens tones and change the ELS configuration of each vehicles with the sound strings and their corresponding name for the audio file provided by @TheHC09, which I had mentioned in the beginning.
For you reference, here are the siren tone name used in the folder, their corresponding ELS audio string and the actual siren in my setup:
For your reference, here are lines that are I have made changes in my xml files, which was used in the demo video:
Police, sheriff and paramedic vehicles: Highway patrol cars Firetrucks Police/ sheriff bikes Highway patrol bikes At this step, you have to be sure each lines you've made changes are valid and has no mistakes.
If not, there will not be sirens played in the game, or a specific tone cannot be played in the game.
 
Finally, install the siren tones with the vehicles folder that has its size reduced previously, by using open IV.
 
And that's it, you have just made your own multi-siren setup : )
If this forum post is too complicated for you then I would suggest you to watch this video tutorial by @Doug1637, it is basically the same thing.