You're right, Mikey. They did dub in the Q sound for the Ward LaFrance from season 2 on.
I just watched the pilot movie for Emergency again for the millionth time while I was working one of my paramedic shifts. It's rather funny-- they dub in several different siren sounds for the same unit in different scenes.
In a couple of the early episodes not only do they change siren sounds, often confusing mechanical sounds for units with electronic speakers and vice-versa, but they also change apparatus. In one quick scene, Engine 51 (supposed to be a Crown) parked at a distance is actually a Seagrave. When the camera goes back, it's a Crown again.
I guess they never thought that there would be "us" out there with a sharp eye, a sharp ear and knowledgable about which sirens sound like what.... LOL!
Hollywood.... It's in the Script
John