Any tool enchanted with unbreaking of any level can cause the breaking sound to either play without the item breaking, or break without the sound, at random. This only happens when breaking blocks. It also occurs with the sword.
When a tool with unbreaking finally breaks, it only then makes the breaking sound and then is gone from the inventory.
1. Get any tool, (pickaxe, shovel, shears, etc.) preferably with a durability of 1 for ease of testing.
2. Enchant said tool with unbreaking of any level.
3. Mine the appropriate block for the tool until the tool breaks.
4. Observe that the tool will either break without a sound or make the breaking sound and remain unbroken.