When a baby mob is underwater, you will notice that its ambient sounds are lower pitched compared to their ambient sounds when out of water. The best way to find the sound bug is to spawn baby mobs that sink underwater (example: all of the zombie mobs except drowned).

The example clips shown are with the baby zombie mobs (including baby villagers). But, it also happen with any mob that has a baby. 

The baby mobs' ambient sounds when underwater (notice they're lower pitched when they're underwater):
Baby Villagers sound lower pitched when they're underwater.mp4
Baby Zombie Villagers sound lower pitched when they're underwater.mp4
Baby Zombies sound lower pitched when they're underwater.mp4
Baby Husks sound lower pitched when they're underwater.mp4
Baby Zombified Piglins sound lower pitched when they're underwater.mp4

How they were supposed to sound like (the actual ambient sound pitch that the babies have):
How the baby Villagers supposed to sound like.mp4
How the baby Zombie Villagers supposed to sound like.mp4
How the baby Zombies supposed to sound like.mp4
How the baby Husks supposed to sound like.mp4
How the baby Zombified Piglins supposed to sound like.mp4