It's hard to imagine that 2001: A Space Odyssey would have its incredible cinematic impact without that classical soundtrack. But director Stanley Kubrick originally hired a composer to score the film, only to abandon the score in postproduction.
Kubrick tapped Alex North, who had scored a number of classic films, including Kubrick's own Spartacus, to write the 2001 score. Kubrick deemed the score inadequate for the film, and replaced it with the classical musical tracks that are so familiar today. North wasn't aware of the change until the film premiered.
You can listen to 40 minutes of North's score on rdio to get a sense of his aural vision for the film. YouTuber Robert Beardsley also did a quick and dirty edit to re-marry North's score with the iconic introduction to the film. While this version probably doesn't sync up exactly with what North had in mind, it does give us a sense of how the music and visuals would have worked together. Nothing against North or his music, but it seems in this case, Kubrick was right; this music simply wasn't right for the movie experience Kubrick envisioned.