When responding to a call in the middle of the night, do the firefighters have to sound the fire engine sirens even though traffic is probably light?

Yes, state vehicle code mandates that while responding Code 3, an emergency response vehicle must have all emergency lights on and the siren sounding. A Code 3 response is initiated when life, property or the environment is in immediate jeopardy (i.e. life-threatening emergencies, fire or the release of hazardous materials.)