One reason that film music often struggles to get mainstream acceptance as music worthy of interest on its own is the fact that film music is always dependent on the film for its inspiration and genesis. Film composers don't tend to have a great deal of freedom in their work, and they have to compose their scores to accommodate the timing and rhythm of the film given them to score. Obviously some of these composers are masters in their own right and create great works even with these constraints, but the constraints are still there.
Music as an aid to storytelling is best seen, prior to the movies and even after them, in the world of opera, but there is a smaller arena where the great composers of the 19th century often found themselves dealing with at least some of the same constraints that film composers would later confront. I'm talking about incidental music for plays.
Some incidental music has gone on to be quite famous indeed, seen as masterpieces on their own. Felix Mendelssohn's work for A Miidsummer Night's Dream is one of the enduring classics, and one of its melodies has even become a traditional recessional in modern weddings. Hector Berlioz also wrote incidental music, as did Mozart, Schubert, Schumann, Bizet, and Grieg.
And so did Ludwig van Beethoven.
What's interesting about a lot of incidental music is that, Mendelssohn's work for Shakespeare excepted, the music has outlived the plays in nearly every case. Schubert's music for Rosamunde is well known (and in that case, mainly for the overture alone), but the play is almost completely forgotten. Likewise with Beethoven's fine music for a play called The Ruins of Athens. This play was written by August von Kotzebue for performance at a new theater in Pest, and if it's been performed anywhere in the last hundred years, I'd be shocked. But because one of music's greatest of all masters wrote incidental music for it, the play isn't completely forgotten.
Beethoven's suite of incidental music is one of his more pleasing works, even if even this group of pieces is rarely heard except for the Turkish March movement. The suite is not purely instrumental, featuring movements for voices and orchestra. It's all suitably dramatic but also very much Beethovenian. For all his anticipation of Romanticism, Beethoven was even at his most ostentatious always grounded in classical proportion and form. This suite, with its drama and genial melodies, doesn't so much seem to anticipate Wagner as look back at (and, in so doing, extend the reach of) Mozart, and it does so with the geniality of the Beethoven who wrote the Pastoral Symphony.
I've long maintained that one need not necessarily have seen a film to appreciate its score on a separate listen, and when people question me on this point, all I need do is point to music like Grieg's Peer Gynt Suite or Beethoven's The Ruins of Athens and ask, "Do you really need to see a forgotten play to appreciate this music?"