I noticed that the "Beepbox" extension does not work with the latest version of BeepBox. I suggest replacing it with the actively maintained and far more functional version created by me. This can be found here:
Github Link
Not only does this support jsons, but it's also based on UltraBox meaning it will work with modded BeepBox songs as well as the original BeepBox. This extension also contains much more functionality with interacting, reading, and editing the songs. I'm also an Ultrabox developer so I'd say it'd make more sense to add the extension of the person who actually knows what they are doing.