Setting Off Fireworks in Potions Class
Muffliato creates a faint buzzing sound that prevents other people from overhearing your conversation. The Imperturbable Charm only repels physical objects, but not sound. However, Molly Weasley used the Imperturbable Charm to prevent eavesdropping on Order meetings not by actually blotting out sound, but by preventing Extendable Ears from being slipped through the door.
Originally Posted by Lovemagic
Tim the Enchanter
EDIT: Looks like the Post Nargles have mixed up the order of things again...