I think that Changez's voice is probably the most dominant in the narrative. His voice is the most evident in the novel for a couple of reasons. The first is that he is the narrative voice. It is through Changez that we get to know everything. His voice almost serves as a soliloquy to the reader. For this reason, I tend to think that Changez is the most influential character on the reader. Whatever one feels about the novel at its end, it is because of Changez. This is where the reader reaction strikes.
I would also suggest that even though character voice might not be as prominently placed as Changez, these individuals are not silent. While these characters do not speak as much as Changez, they do speak to the reader, casting an impression that is present. Erica's presence is one that speaks to the construction of Changez's emotional affect. When she dies, when her voice goes silent, there is little chance of him turning back. Even the American "speaks." I think that he speaks to what the reader would perceive regarding the threat of terror. If the reader sees antagonism between the West and those from Pakistan, then the American speaks to a condition of adversary. I think that this is where he might be silent, but his presence is audibly felt in the understanding of the narrative. While these characters do speak, I believe that Changez's voice speaks more throughout the understanding of what is rendered for the reader.