The trait of being acutely aware and extra sensitive can be a funny combination and can sometimes work against us.
When we use the term sound sensitive we generally mean that when a sound occurs, the dog is sensitive to it and has a negative reaction,
usually fear in the form of shaking, drooling, trembling and hiding. Often when a thunderstorm occurs, the loud claps of thunder are what set
the dog off. Fireworks are also a loud noise that has been known to affect dogs as well as any other loud noise such as gun shots and cars back firing. It does not have to be limited to loud noises, sharp pitched noises can bother dogs as well such as smoke detectors, fire alarms, and some ring tones as an example.

Being sound sensitive is not really something your dog is born with, while they are predisposed to being more sensitive than other breeds simply by being Aussies, the behaviours that they develop are a combination of learned behaviours from watching how other animals within the home or the humans react and an unlearned behaviour from dogs who have never been taught how to deal with stress in a reasonable and calm fashion. These behaviours will develop over time. Your puppy at 8 weeks old from me is NEVER EVER going to be sound sensitive because I work with it and expose it to many different things that could set it off and I reward brave behaviours. We have found that dogs who are living in single animal families are more likely to be sound sensitive than dogs living in groups, the reason is because the older dogs who are already confident will pass that confidence on to the younger dogs in the family and the younger dogs will never think loud noise is a reason to be nervous. The only exception is if the older dog is already sound sensitive and reacts negatively, then that dog will pass the fear onto all of the younger dogs who come in after wards.

So what can you do to help prevent your dog from being sound sensitive.
When you bring your puppy home you want to expose them to loud noises slowly and gradually, sit out on your porch during a thunder storm and bring out the puppies bowl of food. Let him associate eating with noises. Teach your puppy to do fun things like tugging and fetch, play with the puppy during the storm and make sure he is having fun. If a loud crack of thunder happens and the puppy startles at all, ignore the reaction, DO NOT in any way coddle him, instead redirect him to playing and having fun and take his mind off of the loud noise. If you do not make a big deal about it, he won't either. The key is to start as young as possible. As a breeder in the summer if the weather is nasty and we have loud thunder, I will open all the windows and if possible take the litter of puppies out onto the covered deck, when they see that their mom, the other dogs and the humans are not phased by the loud noises, they quickly settle and are not bothered by it.

You can do other things as well in day to day life that do not have to do with noises. The reason a dog startles at the noise is because he can not
identify where it is and if it is a threat or not, he needs to learn to cope with stress. You need to watch how you interact with the dog in stressful
situations and help build their confidence and coping skills instead of coddling them and making it worse.

If you have an older dog and you notice that he is starting to be bothered by loud noises you are at a tipping point. You can try working with them and encouraging brave behaviours through play in the house where the dog is secure, DO NOT have a dog who is in any way nervous of loud noises outside off leash, too many dogs bolt and end up never being seen again.

If your dog is already at a high level of fear with hiding, shaking, drooling, often seeking shelter in a bedroom under a bed or in a bathroom bathtub then you are likely beyond training. There are homeopathic remedies such as Rescue Remedy that you can try, Thunder shirts that the dog can wear work on most dogs and in extreme cases you may need a veterinarian prescription.

The best way to avoid having a sound sensitive dog is to purchase your puppy from confident parents who are not passing on fearful genetics and starting to work with the puppy right at 8 weeks old to get them used to loud noises and teaching them how to deal with the stress of everyday things like noises.