Why Do Dogs Howl?

I have 3 dogs, all of different breeds. Each of them is unique in their own way, endearing and peculiar at the same time. One of the unexplainable behaviors they have all developed is the howling at sirens or noises they hear which we as humans may not. I know that howling at the moon or at sirens is something many dogs do, but not all dogs have this behavior. Howling has been researched and some believe it is an ancestral behavior - likely from wolves. But one has to wonder why in this day and age that which drove wolves to howl is no longer necessary in the domesticated house dog. So one would wonder if the noises the dog hears hurts the ears and howling is a reaction to pain. But this theory is not supported by veterinarians who believe that sirens and piercing noises do not hurt a dogs ears.
The mystery in my household occurs daily in the wee hours of the morning in the bedroom when one of my dogs raises his head with nose pointing to the sky and begins the howl. I call him the tenor of the group. Like a chorus the second dog will join in, this one more like the alto of the group. Then, finally the third one joins in. He really sounds like the soprano of the chorus. It would be amusing but in the wee hours of the morning. I listen for a siren, and it is not obvious to me. So was it once a siren and now it is a habit? This musical chorus lasts for a few minutes, then the dogs go back to sleep. It takes a bit longer for me to get back to sleep.
Occasionally during the day I will hear a siren, but no howling. And occasionally I will hear a siren and the howling begins. Why? And if I start to sing in a high pitched voice I may be joined by my animal chorus - joining in the same order - the tenor, alto and soprano!
So do my dogs think they are communicating with the lost sound of the distant siren which to them may sound like another dog howling? It is a thought. And when they "sing" with me are they positively rewarding me? I do think I won't be able to break them of the habit of wee hour morning howling, so I just need to accept it.