The ears of puppies open when they are about two weeks old. Until that point, the ear canals are sealed. Because of this, puppies are functionally deaf when they are born.
Because the gestation period for dogs is relatively short, many parts of puppies' bodies, including their ears, are not fully formed when they are born. Silence is necessary as the puppies' ears are developing. If noise enters a puppy's ears, pressure changes from sounds could damage structures inside of them.
Typically, a puppy's hearing reaches its full development a week after the ears have opened. Puppies' eyes also open after about two weeks, but they take longer to reach their full function.