Like stanhope said.
For the Idap Ambulance from the Laws of War DLC put this in the on activation field of your trigger:
nameofyorambulance animateSource ["lights_em_hide",1]; [nameofyorambulance,"CustomSoundController1",1,0.2] remoteExec ["BIS_fnc_setCustomSoundController"];
change "nameofyourambulance" to whatever you have named the ambulance