Even before McMurphy is actually seen by Bromden and the other patients, his voice his heard, and it is clear that the way his voice is described says a lot about the kind of character that he is and the impact he will have on the ward. Note how Bromden describes McMurphy's voice:
Talking louder'n you'd think he needed to if the black boys were anywhere near him. He sounds like he's way about them, talking down, like he's sailing fifty yards overhead, holldering at those below on the ground. He sounds big. I hear him coming down the hall, and he sounds big in the way he walks, and he sure don't slide...
McMurphy's voice is one of the many ways in which he is presented as a character who, unlike patients such as Bromden, refuses to let others dictate who he is and how he should act. Note the description given above of how his voice makes him sound superior to the "black boys" who are trying to chase after him to take his temperature. In particular, for Bromden, who imagines himself to be so small, McMurphy's voice makes him sound "big." The impact of this voice on the ward is to begin to make them think that they can become "big" as well, and that they don't have to accept the way they are treated in this environment.