Voice refers to the guiding sensibility or consciousness through which we see view a story.
Voice has a great deal to do with the characterization and consciousness of the narrator (or narrators). The voice of the narrator mediates events for the audience and colors the story with a specific sensibility. Voice, like perspective and point of view, may change and shift from character to character throughout a narrative.
"They all laughed again. Stephen tried to laugh with them. He felt his whole body hot and confused in a moment. What was the right answer to the question? He had given two and still Wells laughed. But Wells must know the right answer for he was in third of grammar. He tried to think of Wells's mother but he did not dare to raise his eyes to Wells's face. It was wells who had shouldered him into the square ditch the day before because he would not swop his little snuffbox for Wells's seasoned hacking chestnut, the conqueror of forty. It was a mean thing to do; all the fellows said it was. And how cold and slimy the water had been! And a fellow had once seen a big rat jump plop into the scum."
(Stephen Dedalus as voice)
-James Joyce, A Portrait of the Artist as a Young Man