Howling is a completely normal dog behavior. Just like humans, dogs use vocalizations to communicate to others – both animals and humans.
Sometimes they howl when they hear a high-pitched sound such as sirens, music, or a baby's cry.
If your dog howls excessively, see your veterinarian to rule out a medical problem, such as pain. If there is no medical reason for howling, then perhaps your dog is just seeking attention. Your vet can advise you on how to stop it or refer you to a behavior specialist.
And if you have an overly vocal dog, it's always important to reward your dog for being quiet! Give treats and praise when your dog is behaving to encourage calm behavior.