On PS3, get into a police car, turn the lights on, exit the car whilst turning the car off, (on ps3 you hold the triangle, until the cars ignition is turned off) then get back inside the car, the second your inside, press pause menu, the lights will still flash, but no sound.
I hope it helps, although you cant move.. your on pause menu and the second you resume the sound comes back on.
As far as i know there's another way and that's to destroy the engine and the siren will turn off but lights stay on, thus leaving you in the same predicament!
I dont own a xbox so i dont know if its the same but theres my 2 cents