Hi there!
You just got your new fancy Police Siren Soundpack, and somehow it doesn't or only partially work? BEHOLD!
You'll need to rename your .wav Files according to the table below, for your police sirens to work again.
Mentioned table:
I'd recommend renaming your files while installing the Siren(s), i.e. dragging "0x0D329446.wav" onto your Desktop -> renaming it to "SIREN_PA20A_WAIL.wav" -> moving "SIREN_PA20A_WAIL.wav" into your vehicles folder accompanied by your vehicles.oac
Enjoy.