The easiest way to do this is create a camera region that covers your scene. Scroll down the Property list for the region, and you'll see options for "Default Music" and "Combat Music". These are drop-downs that let you select from all of the existing music tracks in Dead Man's Switch (and Dragonfall if you're using that editor). The game will automatically switch between the tracks as you enter and leave combat.

If you want to start playing music in response to an action, you can create a trigger with the DO action Gameplay -> Play Music.

Note that, in both cases, you can only play tracks already bundled in the game; there is currently no way to add your own custom music to UGC.

Thanks so much for the help!