The Song of Resonance can only Lost Ark Gold be purchased from a particular NPC known as Treasure Hunter Igran. He's found on the Deck Ship that is located on Pleccia island, which is located in the Sea of Gienah. It's on the west of Anikka.
When you're there head toward the southeast to your Deck Ship and talk to the Treasure Hunter Igran. He'll offer items to buy in exchange for Pirate coins, like an item called the Song of Resonance.
It'll cost you 16,500 Pirate coins, which means it's possible that you'll need to do some work before you can get it. If you don't have enough of them then check your inventory to find Coin Chests. The opening of these gives you a chance of earning several kinds of Coins that you can convert to Pirate Coins. You can talk to Traveling Merchants throughout ports. If not, you can utilize the general ways of getting more, like quests and daily shipments.
Once you're there, head to the southeast towards the Deck Ship and talk to the Treasure Hunter Igran. He'll offer items to buy in exchange for Pirate coins, like the Song of Resonance.
It's priced at 16,500 Pirate coins, which means buy Lost Ark Gold you may need to perform some farming before getting it. If you don't have enough of them Check your inventory for Coin Chests. When you open them, you have an opportunity to earn several kinds of Coins which you can exchange to Pirate Coins. It is as simple as talking to Traveling Merchants who are scattered throughout ports. Otherwise, you can use the common methods to get more, like quests and daily shipments.