As I was testing an issue with the credits music (please see issue 40275), I happened to stray idly by a witch. Since I had the development console open next to me, I immediately saw an error which said:
"Unable to play unknown soundEvent: minecraft:mob.witch.idle"

I tried hitting the witch, and not only did I hear no sound, but I also got:
"Unable to play unknown soundEvent: minecraft:mob.witch.hurt"

Upon examination of the /assets/minecraft/sounds/mob folder, I discovered that there was no directory for witch sounds! How did that happen?