What nostalgists now call Old Time Radio (or TV without pictures) is the forgotten mass medium of the first half of the 20th century. Radio was the nexus of three forms of mass entertainment: the movies, pop music, and TV. In the USA, Hollywood was a center of radio programming (along with New York and Chicago) and many artists worked in both media, sometimes crossing over from stardom in one to the other. The national radio networks basically morphed into the TV networks, such that early TV also had a lot artist crossover and many formal similarities to radio (the announcer, the sponsor, even specific programs). In other countries, such as Britain, radio drama and comedy have had a much longer lifespan.
I have the impression that little is known about the place of "soundtracks" in Old Time Radio, by which I mean that I know little about it. Try Googling "radio music" and see what you get. My memories of listening to tapes of comedy programs (Jack Benny, Fibber McGee, etc.) imply that perhaps there was a signature theme and transitional music to indicate a scene change, but that's it. Some programs certainly used themes, such as the William Tell Overture for THE LONE RANGER, or Flight of the Bumble Bee for THE GREEN HORNET (notice that these aren't original compositions).
I received a box of THE SHADOW LPs as a child (and don't think I ever finished listening to it), and I don't really recall music being as vital a part of the show's identity as the human voice, whether by announcer or actor. Perhaps this is largely the way things were.
It's odd when you think about it. You might suppose music would be crucial to radio drama, an audio medium. Music's function in film, which we reasonably classify as a visual medium (though it is an audio-visual medium, even in the silent era), is to elaborate on what we see and what we don't see. However, the exclusively audio nature of radio, in which we see nothing except in the imagination, might mitigate against the use of music when a writer is heavily using expository dialogue which the listener must hear clearly ("Got to climb...up....this trellis!" "Ah ha, I'm invisible to you, now that I've clouded your mind!") and when announcers and narrators shout everything at us.
And yet, music clearly had a place. For example, most of us know that Bernard Herrmann, after working with Orson Welles for MERCURY THEATRE, came to Hollywood and became music director for COLUMBIA WORKSHOP on CBS. The Wikipedia listing for this program includes a complete listing of the broadcasts, with composer information where available, and it contains so many jaw-dropping items of ambition and intrigue that it would be tedious to summarize here. Please go look it over, and you'll notice many items of fantasy, horror and science fiction; programs not only scored by Herrmann but Leith Stevens and Van Cleave, and programs from Benjamin Britten, John Cage and Virgil Thomson. At the bottom of the page, it mentions that someone has written a Ph.D. dissertation on Herrmann's work for this series. And this page offers 63 public domain audio files from this program. This page refers to a follow-up series, CBS RADIO WORKSHOP.
I'm guessing that if music was important to radio shows, its best shot was on anthologies devoted to horror and suspense, but I've never heard much about the music for LIGHTS OUT, INNER SANCTUM, SUSPENSE, QUIET PLEASE, etc. We rightly think of these as writer-producer programs (Wyllis Cooper, Arch Oboler, Norman Corwin) that relied heavily on actors and sound effects. Did music have a particular impact on these shows? Or was being music director a thankless task? Books like John Dunning's "Tune in Yesterday" make no mention of the music on such programs, but that may be par for the course.
The research into music on radio might be hampered by the fact that, while plenty of broadcast recordings exist, I must suppose that few if any separate music tracks exist. I surmise that radio, being largely a live medium, simply played the music live during the broadcast, and that separate tracks probably don't exist. I'd be grateful for information from someone more informed. | 
