Well, funny story. An EMS supervisor (IIRC) used to drive around talking to his wife on his cell phone the whole shift. So two innovative medics taped the PA mike open one day while he was out of the zone car, so everything he said all day was blasted out over the PA while he was driving through the rather crowded city streets.
At LifeCare, we used to turn the siren switch all way to "Hi-Lo" when other crews would forget to lock up their ambulances at UVa. ("Hi-Lo" is the two-tone siren I like to refer to as "French police siren" 'cause you always hear it in the movies during European chase scenes.) UVa MedCom has a tendency to complain if an unit blows sirens within sound distance of the hospital....
Scott
P.S. Probably best to consult local and state laws where you are about PAs in a POV. In Virginia, you can have one of those electric "airhorn" sounds, but not sirens in a POV with PA. There are probably good reasons you might need to use a PA in a POV. But, abuse can land you in hot water even if it is legal where you live....