The answer is high frequency
The answer is that the dogs are responding to a higher frequency than we "humans" can here. Not all sirens produce this higher pitched sound but most do. The sound is about the same as a "dog whistle". Blow a dog whistle and you may have the same results.
I don't think it "hurts" their ears but it may bother them a bit if they have infections. I've never seen dogs get vicious as a result of hearing this noise so I don't think the howling response is terriotorial. It may just fall along the lines of "yawning". You see someone yawning and you will do it a few minutes later. :rolleyes: :cool: Same with howling.