All-Knowing Singing Narrator

In many musicals, there is a character who acts as the narrator, knowing everything about everyone's business despite the fact that he often is a character in the story himself, and shouldn't really be able to know this.

Film - Animated

 * Clopin in the Disney version of The Hunchback of Notre Dame. He establishes this in the first five minutes of the film, considering he opens it with the story of how Frollo killed Quasimodo's mother and 'adopted' Quasi, despite the fact that he wasn't even there at the time and the two (or possibly more, if you count the staring masonry) who were certainly wouldn't have told anyone.
 * Arguably, Gringoire in the French rock opera.
 * Alan-a-Dale in the Disney Robin Hood
 * The Muses in the Disney adaptation of Hercules

Film - Live-Action

 * The Grave Robber from Repo! The Genetic Opera of course, he's reading a newspaper about the events of the Opera, so even though he wasn't there he still knows, unfortunately, however, they don't do the same when explaining how he knows all about Blind Mag's contract problems.
 * He hands the newspaper to Shilo. Not to mention, Mag's rival is standing right there at the time.
 * The Criminologist in The Rocky Horror Picture Show.
 * In a non-musical version, the Radio DJ from The Warriors mostly fits into this category.

Literature

 * A book of Greek Mythology featured a script-version of the story of Achilles. "Chorus" is described as the "Narrator who knows everything".

Live-Action TV

 * Used (and Lampshaded) in "The Day They Shot Agarn" episode of F Troop.

Theatre

 * The revolutionary narrator from Evita, who is sometimes called "Che".
 * The Narrator in Into the Woods.
 * The Narrator in Joseph and The Amazing Technicolor Dreamcoat.
 * Officer Lockstock and Little Sally in Urinetown
 * The urchins/Ronettes in the musical version of Little Shop of Horrors.
 * The Balladeer from Assassins.
 * The Leading Player from the musical Pippin.
 * A non-singing example: the Stage Manager from Our Town.
 * Pseudolus, the main character of Stephen Sondheim's A Funny Thing Happened on the Way to the Forum
 * The Cat in the Hat from Seussical the Musical
 * The Lecturer in the musical adaptation of Reefer Madness
 * The Narrator in Blood Brothers.
 * Lonny from the jukebox musical Rock of Ages

Western Animation

 * Parodied in the Arthur episode "The Ballad of Buster Baxter": "Mom, there's a singing moose in front of the house!"