The jukebox song registry in 24w21b can't regist the music in the non-vanilla resourcepack as Item component, game will report an error when you try to enter the world.

Reproduce steps:
1. Add a music to a resourcepack.
2. Regist this music in datapack.

3.Try to enter the world, it will report an error.

4.Change the "sound_event" in the 2 step to any vanilla sound effects.

5.Try to enter the world, it works successfully.

2024.5.24 20:40 (UTC+8):
According to the pinned comment, the key "sound_event" followed a compound can work with third-party music, but followed a string must use vanilla music.