According to the University of California Santa Barbara ScienceLine, it is likely that a dog howls when it hears a siren because it thinks it is another dog and is trying to communicate with it. Dogs are social animals and have many ways to share information vocally, such as howling.
Dogs also communicate by barking. Cesar's Way explains that barks indicate one of four states: a dog is either bored and wants something to do, spoiled and wants continual attention, confused about current circumstances or afraid of something in its surroundings.
Growling is often an intimidating form of communication. According to About.com, dogs growl for a number of reasons, including fear, pain, protecting a possession and defending territory.