What does it mean to refer to the "voice" in a piece of literature?
The way the writer writes.
Every writer has their own flair; their own unique writing style. Their choice of words, sentence structures, and metaphors are very much their own.
This could also refer to the character's voice; again, practically the same as the above paragraph. Usually, the character(s) has repetitive phrases they use, words the choose, and structure of thought and speech. This influences the reader's point of view and opinion on the character(s).
Hope this helps :)