The Song of Resonance can only be bought by a specific NPC named Treasure Hunter Igran Lost Ark Gold. He's located on the Deck Ship from Pleccia island in the Sea of Gienah. It's on the west of Anikka.
Once you've arrived, head toward the southeast to the Deck Ship and talk to the Treasure Hunter Igran. He'll offer items to buy in exchange in exchange for Pirate coins, including and the Song of Resonance.
It'll cost you 16500 Pirate coins, so you'll likely need to do some work prior to purchasing it. If you do not have enough look through your inventory for Coin Chests. Opening those gives you a chance of earning several kinds of Coins that can be converted into Pirate Coins. Simply talk to Traveling Merchants throughout ports. Or, you could use the normal methods of gaining more, including quests and dailies.
Once you're there head southeast of toward the Deck Ship and talk to the Treasure Hunter Igran. The Treasure Hunter Igran will offer you items in exchange in exchange for Pirate coins, like and the Song of Resonance.
It'll cost you 16,500 Pirate coins, meaning you might need to do some farm work prior to getting it. If you don't have enough make sure you check your inventory for Coin Chests. By opening them, you stand an opportunity to earn several kinds of Coins that you can convert into Pirate Coins. You can talk to Traveling Merchants Lost Ark Gold for sale, who are scattered in ports. Or, you could use the standard methods to earn more, such as quests and dailies.