I've listened to some music tracks in several scenes; clearly, the other scenes have their music locked and I must pay to unlock it! Or is there some other way?

Not all scenes have music associated with them. In fact, most do not.

If you don't hear music in a scene that does have it the music might be stopped. In the bottom left corner of the scene description you can find a button that can be used to turn the music on or off.

You can also go to your settings from the bottom menu to turn auto play on or off for all scenes.