Repeated gunfire around any animal is going to effect their hearing.
In the case of hunting dogs, they are very seldom in front of the barrel, where most of the sound comes out.
Most hunting dogs will be a good distance from the hunter when he shoots, such as bird hunters shooting into the air, would tend to throw the sound away from the dog on the ground. I would think that chasing dogs would be in the most danger, such as beagles chasing rabbits, where they would sometimes be a short way from the rabbit when the hunter shoots.
I think it is only smart to keep your dogs away from overly exposing them to shooting practice, or can killing, as we call it around here, but one must realize that if you are going to hunt the dogs, they are not going to be protected in the field.