If you use the number keys to switch a sword to an open spot on the hotbar, you will be able to break blocks with it in creative mode. However, no sound is played when blocks are destroyed this way.

Steps to reproduce:
1. Place a sword into any slot on your hotbar, with nothing else in it
2. Hover over the sword with your cursor and press a number key other than the slot it is in
3. Close the inventory and select the sword
4. Swing at any blocks in range.

Updating the sword's slot at all will reset it, and you will no longer be able to break blocks with it.

I'm sorry if this has already been reported, so many people have reported the inability to break blocks with a sword as a bug itself that it was nearly impossible to search.