I agree with lizardlady. It's one of those reasons. I didn't used to have dogs that howled at sirens, then I got my first dog that did, and it taught my other dogs to do it, and those dogs taught my other dogs, and son and so forth... So, ever since then, my dogs have howled at sirens, even ones I can't hear, but they can.

They're so funny: I have 3 dogs right now, and all of them do it.

The funniest one is Sophie; she's a terrier/chihuahua mix, and when she hears a siren her howl is SO hilarious: it's this really low-pitched, h-rOOOOOOOOOOOOOOoooooo.......... Like a really mournful wolf.
My other small dog, Patches, she just does this really annoying, high-pitched yelp/bark/howl thing. I really don't know what to call it.
And my other dog, a beautiful sandy colored (hence the name) German Shepherd, Sandy, she howls once, and only once, even if the siren is still loud.

I love dogs.