Personally, I do the following: I take the music I have, reformat it using NERO (this is not an advertisement) into the ogg format (this is the format for music in Heroes 5), go to the music folder, find a file there, for example, town-necropolis, copy it somewhere safe (but don't delete it, because the music is good), paste the music I want there, and rename it to town-necropolis. And voila, in my Necropolis castle, the music I want to hear is playing. I do this for all castles, battles, and locations, and the Heroes sing for me.