Issue:

If you saved your game inside the Red Hornets Base dungeon or inside the Twin Brothers house map, export the save file to a new version of the game and load it. You will receive the following error message:

Loading Error:

Failed to load: audio/bgm/[audio file name]

Cause: 

The background audio files for these maps were modified for the new builds in order to avoid localization issues reported by players. What this means is that the name that was stored inside the save file at the moment when you saved your game on one of these maps is now incorrect, so when you load the game, it searches for the audio file of the current map using the old name that was stored inside the save file, and since it doesn’t find it, the game throws the error notifying that the file was not found.

Solution: 

You can solve the problem by leaving the map. When you do this, the game starts playing the audio file for the background of this other map. Now, save your game and restart it.

You will not encounter the issue again since the data of the current map (which the game was able to find) was used to rewrite the incorrect name for audio in the save file.

If you have any questions about this Issue, please leave them in a comment on this post.