Look at the picture.
The blue square is representing the subtitles that were added before 1.13, and the red square is representing the subtitle that were added in 1.13.
As you can see, almost all of the subtitles added in 1.13 are written as "subtitles" not "subtitle", and causing subtitles to be not shown ingame.
Fix JSON included.
Affected sounds
entity.drowned.ambient
entity.drowned.death
entity.drowned.hurt
entity.drowned.shoot
entity.drowned.step
entity.drowned.swim
entity.husk.converted_to_zombie
entity.illusioner.ambient
entity.illusioner.cast_spell
entity.illusioner.death
entity.illusioner.hurt
entity.illusioner.mirror_move
entity.illusioner.prepare_blindness
entity.illusioner.prepare_mirror
entity.skeleton_horse.swim
entity.squid.squirt
entity.turtle.ambient_land
entity.turtle.death
entity.turtle.death_baby
entity.turtle.egg_break
entity.turtle.egg_crack
entity.turtle.egg_hatch
entity.turtle.hurt
entity.turtle.hurt_baby
entity.turtle.lay_egg
entity.turtle.shamble
entity.turtle.shamble_baby
entity.turtle.swim
entity.zombie.converted_to_drowned
item.axe.strip
coschevi
In addition, the turtle subtitle events are missing the "entity." prefix in sounds.json; i.e. they are written as "subtitles.turtle.x" instead of "subtitles.entity.turtle.x", which is how they are defined in the lang files.