One of my friends has a stud dog that is a master hunter, he has been shot over while hunting so much he can't here. It is a good thing he handles.
If the dog doesn't hear the shrill sound of the whistle or your shouting voice then the once handling dog now becomes a dog that doesn't handle.
If you can't get the dog's attention you can't handle the dog. Plain and simple.
Imagine sending the dog off on a blind retrieve and you can't get the dog to sit down and look at you for handling because he/she can't hear your whistle/shout...