Oddly enough, the sound effects are still stored in the files. They're just unused.
How to fix: sounds.json is wrong and one of the slime sound events say "mob.attack". Change it to "mob.slime.attack" and it should work. That's it. Only change the sound event name. I put that in a resource pack and the sound works now.
Oddly enough, the sound effects are still stored in the files. They're just unused.