I tried to install a siren mod to change the siren tones, now when i go to turn on the sirens theres no sounds. nor is there a horn for my police car.. How do i fix it or get it back to the default siren?

Does it work with the backups you made prior to modding your game? 

 

Your thread has been moved to the GTA IV Support Requests forum, as it is not an LCPDFR support request.

Your sirens are muted because the sound file became too big, thanks to the siren mod. Just place your back-up (or the original file wich is outside the mods folder) back and you should be fine.