"My dad's kitty makes no noise at all... opens his mouth and meows, but nothing comes out!"
As you probably know, cats can hear sounds far beyond the range of human ears. It is quite possible the cat is speaking to you, but your pitiful human hearing is not up to the task!
A possibility! Perhaps he is just not saying nice things and this is how he gets away with it! ... except - he used to make noise... then it just got fainter and fainter.