Splash potions of any type don't make a sound when thrown if Friendly Creatures in the sound options is set to 0, also the sound is lower if you lower the Friendly Creatures to a number not 0.
To Re-Create : Use latest Snapshot, and set Friendly Creatures sound to 0 or low and throw a splash potion of any type.
Linked issues
Comments 2
Duplicate of MC-41865.
That's because the sound events of many non-mob entities belong to the Friendly Creatures category. I discovered this when I was looking through sounds.json when creating the sounds.json wiki page.
So it's not really a bug; the developers just stuck all the non-mob entity sounds into the Friendly Creatures category because they didn't know what other category to put them in. Those sounds don't really fit into any category.