We had a cat that became deaf when he was about 15 years old. His meows became louder and like your cat, he was unresponsive to soudns. After a bit, we learned to hear the difference in the tone of his meows...one for love, one for food, one to go out, one to come in....once we figured out the tones, everything was fine. :)
Fester passed away last year at age 18. I still hear his meows from time to time!