It's almost always wrong to assume that the 'voice' that tells a story or a poem is that of its writer. Much more often, the writer has made up an imaginary teller of the story or poem called a persona. The technical term for the 'voice' is a narrator - in a story this is the narrative voice; in a poem it is the poetic voice.

In a story, the storyteller is usually either that of one of the characters within the story - this kind of voice is called a 'first person narrator'. A second common type of narrator is that of a voice - which seems to be that of the author - spoken from outside the story this is called a third-person narrator.

It is important to remember that this 'narrative voice' need not actually belong to the author (even though the author, naturally enough, created it). Authors and poets can adopt what is called a persona to tell their story or poem. This allows, for example, an adult writer to adopt the narrative voice of a child, and so on.