I think I ran into the same issue with using the load() function to load resources in the exported game. I found a fix for it here: https://www.reddit.com/r/godot/comments/13wjo7p/scenes_not_loading_after_export_for_godot_v402/
It looks like Godot uses a “remap” system in exported projects, which renames resource paths from “.mp3” to “.mp3.remap”.
The solution I found was to use ResourceLoader.load() instead of load(). I’m not sure why load() doesn’t do this automatically.