It has to be a cue written for a movie within the movie you're watching.
Oh. The "movie within the movie" bit got past me. I mistook it for a whole-phrase typo, the kind that happens when you re-write a sentence but fail to delete all the trimmings. Forgive my trespasses!
Since this thread got bumped, I use the chance to add the scene I was talking about up there from The Truman Show.
It's a nifty one-two punch, as we see how an emotional moment is artificially build, and the viewers of the show hear the underscore for the show, yet at the same time that moment is an actual emotional moment (because we feel with Truman) and the diegetic source music within the fictional TV show also doubles as actual underscore for the scene.
Leonard Rosenman "Rebel Without a Cause" - the Planetarium cue.
Bernard Herrmann "Where Are You" - there is a cue written for the scene in which the protagonist watches a film in the empty theatre. Brilliantly, the suite from the score begins with this track.
A great example of source music combined with dramatic underscore is the "Death of Fiona" cue from Thunderball. Bond is dancing with the villain Fiona to a cha-cha version of "Mr. Kiss Kiss Bang Bang" when he spots one of Fiona's henchman pointing a gun at him, partially hidden by a curtain. At this point the bongo player, King Errisson, starts a frenzied solo (bongo fury!) while Barry supplies the suspenseful brass accompaniment, building towards the shot--when Bond turns abruptly, the shot kills Fiona, and the music instantly is back to the cha-cha.
unrelated: Bongo Fury was the name of a 1950s album, possibly exotica. I saw it in a record store a few years ago. Don't remember the artist. The album or maybe just the title must have made an impression on a young Frank Zappa, for in the mid-seventies he wrote a song of that title--sung by Captain Beefheart--and then titled the album Bongo Fury.
...name some source cues on albums that actually function as score cues for a movie within the movie you're watching (either written by the composer of the movie you're watching, or by someone else)
Does Parallax View's brainwashing film music count? You people terrify me with your thread specifics, so I bow, scrape, and profusely apologize if this cue doesn't fit within your hallowed parameters.
Batman Returns has a pretty good one. The transition from Danny Elfman's source chime for The Penguin's umbrella to the percussion of the actual score in The Children's Hour always gives me chills. Good stuff.
Very few of you who posted after the bump actually read the original parameters:
IT HAS TO BE SOURCE MUSIC FOR A FILM WITHIN THE FILM!
But the headline of the thread can be confusing, so if one doesn't read first posts properly, or the thread, this delineation may be lost.
One that hasn't been mentioned so far is "Frank's Promo" from Elfman's SCROOGED. It's only a few seconds long, but Elfman is channeling TERMINATOR. See from 0:45 here: