When does McMurphy discover that the Chief can hear?
McMurphy recognizes that the Chief can hear and understand when he gets his support for watching the World Series. Desperate for a majority vote, McMurphy campaigns to the Chronics. He goes to the Chief, who shows and voices his support by raising his hand. The Chronics do not understand what McMurphy is doing, but the Chief does and hear him and his vote indicates as such. McMurphy is so driven by the need to get the votes that he does not stop to realize that the Chief can hear him, but it becomes clear that the Chief can hear.
When McMurphy is seeking help to get his fishing expedition off the ground, he becomes cognizant that the Chief can hear. The Chief does not want to let people know that he can hear and understand. However, McMurphy is rubbing off on the Chief and as his hallucinations decrease, he credits McMurphy's spirit as becoming a part of his identity. It is in this where the Chief, almost unknowingly, speaks to McMurphy and from this, it becomes clear that McMurphy is aware that the Chief can hear. He tells the Chief that he wants him to assume the emotional condition of his physical size both in character and in recognition of his strength. From this exchange, McMurphy recognizes that the Chief can hear.