Custom music not playing

-# this is the 3rd time I'm trying to make this post, istg if the bot ignores this one more time
There's a custom Minecraft map I'm a fan of and so I wanted to make a resource pack for myself that takes its soundtracks and folds them into the background music rotation.
I have added screenshots of what I have so far - the resource pack contents and the sounds.json contents.
From my understanding, this should randomly play the 3 soundtracks while I'm in The End, in addition to C418's "The End".
However, the vanilla "The End" soundtrack is the only one that ever plays. What am I missing?