how does Gregor's strange chirping voice cut him off from communicating with others?

Mostly people are creeped out by it and hardly understand him. Before his metamorphosis, Gregor is alienated from his job, his humanity, his family, and even his body, as we see from the fact that he barely notices his transformation. In fact, even his consideration for his family seems to be something alien to him, as he barely notices it when he loses this consideration at the end. After his metamorphosis, Gregor feels completely alienated from his room and environment and, as a symbol of this, can't even see his street out the window. The Metamorphosis, then, is a powerful indictment of the alienation brought on by the modern social order.