Our mum certainly had hearing problems a few years before Alzheimer's kicked in. Her hearing has naturally got worse over the past couple of years but I'm not sure that it's always that which causes the constant need for repetition of almost everything we say - I think she simply has problems comprehending anything.
I've noticed that she always jumps whenever there's a knock, or if a door shuts, or if I bash plates about in the kitchen. She can hear sounds, but can't understand most of them. Same with her vision; I was becoming convinced that she was starting to lose her sight. She would often ask where I was even when I was sat next to her, but a simple "follow my finger" test from her GP didn't show any problems.
I think JanS is right, it's a processing fault. The brain is such a treacherous thing.
It can be useful. I can swear to my heart's content.