Then, in the advanced editor, you can conveniently select the "music" item from the drop-down menu, click "add," and enter all the information (primarily the path to the music). This is similar to how you would handle other objects like "music."
After all these steps, you just need to assign the music to where you want it to play. As you correctly noted, heroes have "adventuremusic," but that's not what you want; it doesn't play during battles. There should be a separate option called "battleMusic" or "CombatMusic."
But I can't give you a precise answer on this point because I don't remember the exact details.