Where does the authorial voice come into the text and what is the voice saying in Hamlet?
Concerning authorial voice or intrusion into the text of Shakespeare's Hamlet, I'm afraid you're pretty much out of luck. Hamlet is a play, not a work of fiction or narrative poetry. Every word spoken is spoken by a character, not by the author. Therefore, no authorial voice exists in the play.
There's not even a chorus that one might suggest provides authorial intrusion.
I'm afraid the best you may be able to do is to point to the advice the character Hamlet gives to the Players about acting, after they arrive in Elsinore. Some commentators have seen Shakespeare's advice in Hamlet's advice. Even this, though, is speculation, and the text is still technically dialogue spoken by a character, not authorial intrusion.
The closest you will find to an authorial voice is Horatio. He is the prime observer and witness to the events in Elsinore. He provides the backgound of King Hamlet, King Fortinbras and Prince Fortinbras in the first scene. This role become more clear in the final scene as Hamlet leaves to Horatio, "to tell [his] story." In front of Prince Fortinbras now the new king, Horatio establishes himself as the one who authors the play of Hamlet and directs that "this same be presently performed." And so it is.