The Harp of Lullabies has two uses. Firstly, it can be used to confuse an enemy. Question marks will appear above an enemy's head, and they will not be able to cast spells upon you. Secondly, and more importantly, it can cause enemies to fall asleep. Zzzzs will appear above the enemy's head. This is the perfect time to command your pet to attack or to go in for melee attacks.
Deaf creatures are immune to all instruments, as they cannot hear the charming effect.