I used to say that police work would be a lot better with a soundtrack. And safer, when the ominous music rose you'd know that you were in for something serious. While you occasionally hear people refer to the music they listen to as the soundtrack to their lives, it really isn't the same. For the most part our music, while perhaps important to us and not just background music, doesn't function the same as a soundtrack. For a good soundtrack enhances, or even sets, the mood of a scene. Music can be a powerful tool in filmmaking.
I own several soundtracks in my music, mostly because they can make for some great compilations. More often now, in the iTunes world of today, I just cherry pick from them. I increasingly discover that I already have some of the music already. The other day we watched Ghosttown and as the credits rolled the music had me thinking I'd need (I love that – need) to look up the soundtrack. The song was a Wilco song, and I had it. Hmm, can't be because I have too much music. No, that can't be it, that would be like having too many books.
Several years ago, after watching Truth or Consequences New Mexico I thought to myself, man I've got to get that soundtrack, but was never able to find it. On ocassion I'd look for it but it seems it was never released. The other night I just looked up the songs on the internet and went looking for them. I must have been in a way different space then, because it sure wasn't the powerful album I'd always thought it was (although I did already have a couple of the songs – notably Lucinda Williams' and Van Morrison's songs). That just may be another aspect of soundtracks, sometimes the music is more as a part of the film than it is on its own.
The job of chosing a soundtrack generally falls to someone other than the writer. There are of course exceptions, writers/filmakers such as the Coen Brothers or Quentin Tarantino feel that the musical choices are an integral part of the writing. But they also make the films, giving them alot more freedom. Generally it is the job of someone else, and someone skilled at it can greatly enhance the film.
When I started Ghosts of Vimy I had no illusions that one, it would actually become a film, and two, if it did that I would have any choice in any soundtracks. Being a period piece it also changes the nature of the soundtrack. I did/do though have a sense of what I'd like. I don't know if you are familiar with the movie Cal but the soundtrack is by Mark Knopfler (who has done a few music scores) and I imagine a similar treatment for the movie that plays in my head, but with a highland flavour rather than Irish.
In the screenplay though there are three, no make that four, spots that I suggest music. Two of them can't be considered "soundtrack" per se as the music forms part of action (bagpipes). But two of the suggestions are songs that in my mind fit so well that I can't imagine a better song for the background of the particular scene. To me they do exactly what a good soundtrack piece should do, fit the scene seamlessly and set the mood for what is unfolding.