The filename in the BGM folder is “[World of Ennorion 
] Fantasy D&D & RPG Event Music ｜ Sad, Sombre ｜ 1 Hour ｜｜ Mourning the Fallen.ogg”, which I assume is a rip of this Youtube video. As it seems to just be an hourlong loop of another song, according to the video description, I don’t think a lot of work would be necessary to get a more reasonable loop.
I haven’t actually run the game yet, just took a look to see if the apparent filesize issue was something easily identified and handled, so I can’t even say if it’s currently used in the game or not.