When changing the volume slider of Hostile Creatures it changes the slime blocks sounds volume. Change Hostile Creatures volume slider to Off and you will not be able to hear Slime Block sounds.
Linked issues
is duplicated by 3
relates to 1
Comments 11
But it's still a bug, problably just a little oversight when someone was coding the sound effect in.
Confirmed for 1.8.1 (MC-75218).
Its because they use the hurt sound for slimes.