Chief Bromden has not really become deaf and dumb; he simply feigns his deafness in order to gain knowledge about the Big Nurse and the orderlies; for, they talk in front of him, assuming that he cannot hear:
They don't bother not talking out loud about their hate secrets when I'm nearby because they think I'm deaf and dumb. Everybody thinks so, I'm cagey enough to fool them that much.
Fully aware that his being deaf is an act, Bromden narrates further in Chapter 2, as he recalls first meeting McMurphy,
But then's when I remember thinking that he was laughing because he wasn't fooled for one minute by my deaf-and-dumb act....
Later, in the first chapter of Part III, Bromden lies in bed the night before the fishing trip and ponders his charade of being deaf, of not "letting on I heard what was being said," and he wonders if he can now act any other way since his pretence has occurred because people have thought he has been too insensate to hear or see or speak about anything. Recalling the words of his father,
Papa says that if you don't watch it, people will force you one way or another, into doing what they think you should do, or...into doing the opposite out of spite,
Bromden's understanding underscores Kesey's theme that society demands conformity and, as Ralph Waldo Emerson expressed it in Self-Reliance, is "in conspiracy against the manhood of every one of its members." Ironically, then, Bromden feigns being deaf in order to defeat what he feels is the conspiracy against his manhood and individuality. So, he speaks in order to warn McMurphy,
"They can't have somebody as big as Papa running around unless he's one of them. You can see that."
He warns McMurphy of the Combine that will use various means to subjugate him.