What does one call the voice that tells the audience a fictional story?
The short answer is the narrator, but how that narrator functions depends on the point of view of the story. Simply put, there are three points of view from which a story can be told: First person, second person, and third person. Third person can be further broken down into Close and Omniscient Third.
First Person POV refers to the "I, we, me, my, mine, us" narrator, often the voice of the heroic character or a constant companion of the heroic character.
Second Person is the "you" narrator, a method of bringing the reader into the story.
And finally, Third Person refers to "he, she, they" narrator, and is the most common choice for fiction.
If the story is in First Person, the person telling the story is more than just a narrator: they could be the protagonist (main character) or some other key player in the story.