How to reproduce

1. Turn the 'Players' volume slider in 'Music & Sounds' all the way down. Keep 'Hostile Creatures' all the way up.
2. Run the following command:

   /summon minecraft:zombie ~ ~ ~ {CanPickUpLoot:1b}
   

3. Throw a favorable item at the zombie such as an iron chestplate, and notice that when the zombie equips the item, a sound doesn't play.

What is expected

It's expected that the sound that plays when a hostile mob equips a new item would play through the `Hostile Creatures` sound source as they are not players. It should also be noted that when a mob equips an armor piece, the generic item pick up sound is played, rather than the armor equip sound.