My Malcolm is a bit deaf, didn't realise until his wifey Thumpa passed away and he stopped running around and doing stuff, thought it was because he was grieving at first but then noticed that you could come into the room and if he had his back to you , you could shout , clap your hands and bang things around and he wouldn't even notice you until he turned and saw you then he would jump and run off.
If you stamped on the floor he would turn around to se what it was, must have been feeling the vibrations.
Now he has his new Wifebun, Blossom, he is much more active especially when out in the garden as she does the listening for him, when she notices something, he looks up as well, it is quite amazing to see how they work together.