The Song of Resonance can only be purchased from a particular NPC named Treasure Hunter Igran Lost Ark Boosting. He's on the Deck Ship from Pleccia island located in the Sea of Gienah. It's situated to the west of Anikka.

When you're there, go toward the southeast to The Deck Ship and talk to the Treasure Hunter Igran. He'll provide items for purchase in exchange for Pirate coins, such as and the Song of Resonance.

It'll cost you 16500 Pirate coins, which means it's possible you'll have to do some work prior to purchasing it. If you don't have enough of them make sure you check your inventory for Coin Chests. Opening those gives you the possibility of earning various types of Coins that can be traded to Pirate Coins. It is as simple as talking to Traveling Merchants in ports. If not, you can utilize the general ways of getting more, including quests and daily shipments.

Once you're there head to the south to The Deck Ship and talk to the Treasure Hunter Igran. He'll provide items for purchase in exchange in exchange for Pirate coins, including an item called the Song of Resonance.

It will cost you 16,500 Pirate coins, which means you'll likely need to do some farming before getting it. If you don't have enough of them then check your inventory to find Coin Chests. They give you the possibility of earning different kinds of Coins which can be exchanged into Pirate Coins. It is as simple as talking to Traveling merchants who are dotted throughout ports Buy Lost Ark Gold. Otherwise, you can use the standard methods to earn more, like quests and daily shipments.