But there’s not just soundtracks, of course.
I’m listening, as I’m writing this*, to The Age of Style, a compilation of 1930s original songs published by Past Perfect in 1999.
Past Perfect is a small independent label that markets remasters of original recordings from the past (roughly 1920s-1950s).

I found out about Past Perfect in the early 2000s, while I was looking for musical references for – of all things – my Call of Cthulhu games. I have a few recordings from their catalog – and they have proved over the years to be both good listening material and highly useful sound references.

The Age of Style is a two-disc compilation of what we could call 1930s “pop music” – fifty tunes from an age in which pop music did not exist as such.

The record is very entertaining, and helps – helps me, at least – defining an aural impression of that age. After all, this is what my characters could probably catch on the radio.
I can drop a musical reference in a scene (remember what the writing course taught you? Insert sensory details – smells, colors, sounds, textures…) and I know what I’m talking about.

I can use different music to define the characters or help me visualize a scene – but for music that’s inside the story, a good reference for me is essential.
Sometimes these songs sound silly, or saccharine, or plain weird – but after all, that’s the interesting part.

And one finds the likes of Carroll Gibbons, and that’s always a treat.

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* Yes, we all remember what Ray Chandler wrote about writers writing with their radio on. So sue me.