Some dogs, like some people, are easily startled by loud or sudden noises. If your dog simply alerts and pricks his ears at a new sound, he will probably be pretty easy to desensitize. A dog who breaks out in over-the-top barking or cowers under the bed will be a bigger challenge. With patience you can teach your dog to accept unexpected noises as part of living in the home.
Visit the vet. If your dog's startled behavior is a new development, he may be losing his hearing. While there is nothing you can do for normal, age-related hearing loss, you can talk with your vet about ways to help your dog adjust.
Expose him to noise, so he learns to ignore it. If your dog is a new addition to the house, or you have recently moved, he may just need time to settle into the new noises, such as traffic. If the noise that startles him is more sporadic, such as fire engines or thunderstorms, record the noise and play it back for him frequently. Start off by playing it relatively low, then once he gets more accustomed to it, you can turn up the volume.
Remain matter of fact when your dog is startled. When your dog reacts to noise, either your recorded noise or spontaneous noise, ignore him. Don't rush over to soothe him, because this teaches him that there is, in fact, something to worry about. You also don't need to punish him. That can lead to anxiety, which will only make the problem worse. If he stays hidden or walks around barking for an extended period of time after the noise, distract him by throwing a ball or doing some obedience work, but give him a few minutes so he has the opportunity to calm down on his own first.
Consider homeopathic options. If your dog continues to be startled by noises, even after you have worked with him, your veterinarian may be able to recommend some homeopathic options, such as lavender oil, or pheromone-based products to relax your pet.