Just to update you, I searched through the support channel of Naninovel's Discord server, and I did find a couple of other devs talking about having this either when testing Mac builds themselves. However, there was no solution actually posted following them asking for help. I have contacted one of the devs who was asking for help on fixing the problem to ask them if they ever solved it themselves, but I have no idea if they will respond to me or even see my message.
I will post something in the support channel myself because the last mention of the issue that I could find was from February this year. Surely someone knows how to fix it by now. Before I post though, would you be able to download the Mac version of the DD music box and just tell me if that has the same problem? I only ask because the music box was created using the most recently updated version of Naninovel, and I was wondering if perhaps the issue is because DD was made in an older 2020 build of Nani and perhaps it's something that has since been fixed by the software dev since.
If the music box works, it just means I would need to update all of DD to the latest version of Nani and make a new build. Though from what I understand, updating may mess up my custom UI and I'd have to spend time putting it all back together again >.< But if it still doesn't work even on the latest version of Nani, that's something I can be sure to mention in my support request.