It is believed that the Song of Resonance can Lost Ark Gold only be bought from a particular NPC called Treasure Hunter Igran. The NPC is located on the Deck Ship that is located on Pleccia island, which is located in the Sea of Gienah. It's situated to the west of Anikka.
When you're there head to the southeast to on the Deck Ship and talk to the Treasure Hunter Igran. He'll offer items to buy in exchange in exchange for Pirate coins, such as The Song of Resonance.
It will cost you 16,500 Pirate coins, which means you'll likely need to do some work prior to purchasing it. If you do not have enough make sure you check your inventory for Coin Chests. When you open them, you have an opportunity to earn different kinds of Coins that can be converted to Pirate Coins. It is as simple as talking to Traveling Merchants throughout ports. In other cases, you can make use of the common methods of getting more, such as quests and dailies.
Once you've arrived, head toward the southeast to Lost Ark Gold for sale your Deck Ship and talk to the Treasure Hunter Igran. The Treasure Hunter Igran will offer you items in exchange for Pirate coins, including an item called the Song of Resonance.