I solved it, it was actually stupid. The ELS .xml file had all the sirens set to false. Everything works now I set them to true.
Thanks anyways.

Oh yes, almost forgot. I installed 'Federal Signal Smart Siren' and noticed this problem later on, but I came back to the stock siren, now it's at 15.6mb. I just noticed the same problem on an add-on car. But all the others work perfectly fine.

Does anyone know why sirens and horn don't work on some vehicles? Such as fbi2, but on the others, it works fine. Also, the lights work flawlessly with ELS but there is no sound when I turn on the sirens or press the horn. Maybe because I have installed a new fbi2 vehicle, but nothing like this shouldn't happen. Any clues, I'll be thankful for any ideas.
Thank you.