After you turn on the siren of one of those vehicles, the sound will be played until you exit the car.
Once you have exited, the siren sound will stop...
+ when you burn/explode the vehicle, it will repeat the sound once more.
Fixed internally.
After a while of playing VC:MP these days, I concluded that it is the best to remain the sounds turned off for the unoccupied vehicles.
I was just watching how many police cars were with the siren turned on, but no sound. Felt better.
If you could create a boolean function for this, it will be the best ! :)
(would look good in RPG servers)