I have a mob farm with a lot of hostile creatures, I turned off the volume completely for the hostile creatures so I would not have to hear them, however, I could still hear the jumping sound of the magma cubes, its only after I turned off the volume 100% of friendly mobs did the sounds stop, this does not make sense as the magma cube is a hostile creature, I don't know if the same would happen with slime, as I have not tried it