Some sounds for a small slime of size 0 seems to have been removed. In game, they no longer produce a sound when they jump in game. The sounds that are affected are:
[19:27:45] [Client thread/WARN]: Unable to play empty soundEvent: minecraft:entity.small_slime.squish [19:27:45] [Client thread/WARN]: Unable to play empty soundEvent: minecraft:entity.small_slime.jump
Confirmed for 1.13.2-pre1. It appears as though they do have sounds (can be seen with /playsound), but they just don't play when the slime jumps and squishes.
Also you get a warning on the console:
[19:27:45] [Client thread/WARN]: Unable to play empty soundEvent: minecraft:entity.small_slime.squish
[19:27:45] [Client thread/WARN]: Unable to play empty soundEvent: minecraft:entity.small_slime.jump