The "voice" in works of literature can refer to two different things. One is the author's style and what makes him unique and distinct. The other is the character-specific speech and thought patterns of the main character.
Every author writes with a certain style that is unique and often flows into the protagonist. These different styles help create characters and set moods in written work. A story might be told in the "voice" of a child or the "voice" of a politician, even though the author is neither. Voices in literature are not to be confused with the voices in writing, which are referred to as active and passive voice.