Although I can't give citations, I remember news accounts on how newer
police cars are placing the sirens on the front of the vehicle to
avoid the problems with communication in the passenger compartment and
temporary losses immediately after using the sirens.
On an interesting sideline, I have heard some departments suggest that
officers fire live rounds outdoors WITHOUT hearing protection at least
once a year. This is in response to a situation where an officer
fired her weapon in self defense and then dropped it fearing it had
malfunctioned. She had just never heard the sound of a gun firing
without plugs and mufflers.