It work on 1.21.1 but you should replace sounds:

You copy your .ogg music:

You paste it in :

And you delete "11" and renome your music "11"

And you paste it again:

And delete "11" and rename your music "11"

Can the custom song be longer than the stock Minecraft music disc it's replacing, or does it get cut off?

How would this work if you wanted more songs than in vanilla

The following helped me:

• Photos should be sized 500x500.
• After uploading the datapack, fully re-enter the world instead of using the /reload command.