I couldn't seem to find anything saying how to change the music itself - is it hard-coded in, or is there some sort of configuration?

All the musics are hard-coded in yes.
You could tinker with the code to add/remove any music you like by editing this file and adding the new music files in this directory.

You can now tweak wich music plays when in the mod's config (it is still restricted to vanilla Minecraft music)