It has been my personal experience that of all the literary tools and terms, voice tends to be the among the most difficult for students AND teachers to define clearly. I’ve heard a range of responses from:
Voice is the way characters talk
Voice isn’t the way character’s talk, it’s about which point of view is used in the story.
Voice isn’t style, it’s the author’s tone
Voice isn’t tone, it’s the author’s style
And lastly, you can’t define voice, but you know it when you read it or hear it.