I just listened to “Drokk: Music Inspired by Mega City One” a soundtrack to a non-existent Judge Dredd film. Really liked it, and was wondering if there were any other “fictional soundtracks”?
Bo Hansson did Music Inspired by Lord of the Rings back in the early 70s long before there were any films out. I bought my copy when it came out in the UK in 1972.
Opening track
And when I looked at his Wiki entry, I just discovered that he died two years ago. 
Grace Slick released “Man Hole” in 1974 (yes, the pun is intentional) that featured a side-long track of orchestral instrumentation for a non-existent movie.
Kind of like a soundtrack: Rome produced by Brian Burton aka Danger Mouse and Daniele Luppi. Vocals on tracks are provided by Jack White of the White Stripes and Norah Jones. Decent cuts.
Maybe meeting the OP:
Neil Young’s album After the Gold Rush was (at least partially) written to be a soundtrack based on a real screenplay, but the screenplay never got made into a movie. So the movie is non-existent, but not exactly fictional, either. (And neither the songs nor packaging of the album refer to it being a soundtrack).
Along the same lines, the great ***Arthur ***album by the Kinks was the soundtrack to a BBC television movie that never actually got produced.
Frank Zappa’s Uncle Meat was “the soundtrack of a movie you’ll probably never see.” There eventually was a movie of that name, but that’s a documentary about the unfinished movie, released over 15 years after the album.
I was going to mention this because I just heard the song over the weekend for the first time in about 30 years. There are no coincidences. Ask the Mayans.
Looking it up, I see that it was a Jack Bruce song from his solo album after Cream broke up, possibly about their demise. Felix Pappalardi, the singer and bassist for Mountain was the producer of Bruce’s album and always involved with Cream. Mountain was short-lived and underrated. I saw them in college and they put on a great show.
Joe Jackson wrote some instrumentals for a movie called Mike’s Murder. It was released with somebody else’s soundtrack but he put the songs on one side of an album called Mike’s Murder, backed with some of his best rockers. Great album that nobody ever heard. There are no guitars credited on it (bass but no lead guitars), which makes it a rarity among rock albums.
Does the Ennio Morricone score for “The Thing” count? Most of the tracks never made it to the film and what did was heavily modified by John Carpenter.
If that qualifies, then so does Alex North’s unused score for 2001: A Space Odyssey
Joe Jackson’s Mike’s Murder album was originally written as a soundtrack for the movie of the same name, though little of his music was actually used for the movie.
That can’t fit the OP. “Veteran of the Psychic Wars” was selected for the soundtrack to the movie Heavy Metal, and the songs “Heavy Metal: The Black and Silver” and “Vengeance (The Pact)” were made specifically for the movie.
“One of Taraak’s blood can do much…”
The album Soundtracks by Passengers [half of U2 + Brian Eno[sup]*[/sup]] is mostly fictional soundtracks, with brief and somewhat silly blurbs about the films. Except that there’s one real one in there. I think that there might have been another one or two projects that were planned but never made.
*They really should have called themselves Ueno.
Not quite what you had in mind, but the before releasing the soundtrack for Hard Core Logo they released a fictional tribute album. Getting really meta, one of the bands that did a tribute (The Headstones) was fronted in real life by the guy who played the protagonist in the movie.
Matt Mays’ When The Angels Make Contact may fit into this category. Although it does have a trailer. (Youtube)
How about some Space Rock from Buck Rogers?
