You step into the Bat Hole, and break the sonar-in-a-biscuit in half. Deafening waves of sound echo throughout the Hole, and you hear the distant sound of tumbling rocks.
From inventory, after the first three times:
Sonar-in-a-biscuit
There's nothing left to do with these. They're probably still useful in combat, though.
In combat:
Sonar-in-a-biscuit
You crack open the biscuit, and waves of sound wash over your opponent, doing 5-7 damage.