Hey! I'm actually the developer of NiN10 😄
The missing music is a known issue, and I believe it's related to compatibility with the BPCore Engine. I've tested a few other games built with the same engine, and they also have no music, so it doesn't seem to be specific to Funkin GBA.
I'll investigate it and hopefully have it fixed in a future update of the emulator.
Also, if you're okay with it, once everything is working properly I'd absolutely love to feature Funkin GBA in the App Store screenshots for NiN10. Seeing your game running on Apple Watch is honestly amazing!
Great work on the project! :)