I don't mind spilling the beans if you're stuck.
The cave that wants the "sound of darkness" is in Heather's Cove. Which is the village that has the Sharkade. It's close to the lower centre of the map above a small body of water.
In order to progress further you might need to defeat the 2nd boss. If looking at Heather's Cove on the map, the cave that's to the bottom right of it is where they are. You have to enter that cave from the south entrance.
Good luck!