That...seems a bit impossible.
Though I'm not so entirely sure of this myself, perhaps there's some scripted file somewhere in the Lunia folder that automatically makes each episode play the one music file in the sound folder. If opened on notepad, you could change the code to play a different file. Which, you can make yourself by putting it in the sound folder for the bgms.
Hell I never even done that or even know if you can even change the code somehow, so again, not so sure.