Summary:
Item drop sound does not play when dropping items from inventory. It only plays if you don't open your inventory. You can compare this sound to armor equipping, which now plays when equipped in the inventory.

Steps to reproduce:

1. Take a stack of some items.
2. Close your inventory and take the item into your hand.
3. Drop items a couple of times.
4. Open your inventory.
5. Drop items.

Observed results:
The sound does not play when you drop items from an open inventory.

Expected results:
The sound will be made when you throw away items and from an open inventory (as armor equipment does for example).

Notes:
The behavior of this sound cannot be compared with Java, since it does not exist, when a player drops items.