It appears the drowning damage sound is specific to the player being damaged, whereas it would be expected that the sound would be applied to all other mobs.
MC-166357
Non-player mobs damaged by drowning do not play the drowning damage sound
Resolved
Invalid
5
(Unassigned)
Confirmed
Sound
1.14.4, 1.15 Pre-release 1, 1.15 Pre-Release 2, 1.15 Pre-release 3, 1.15 Pre-release 4, ..., 20w28a, 20w29a, 20w30a, 1.16.2 Pre-release 1, 20w45a
Retrieved
Hi there!
I can confirm.