The Song of Resonance can only get it by buying it from a particular NPC named Treasure Hunter Igran. He's found on the Deck Ship that is located on Lost Ark Gold Pleccia island, which is located in the Sea of Gienah. It's on the west of Anikka.
Once you've arrived, head towards the east of the Deck Ship and talk to the Treasure Hunter Igran. He'll sell you things in exchange in exchange for Pirate coins, which includes the Song of Resonance.
It'll cost you 16,500 Pirate coins, meaning you might need to do some farming before getting it. If you do not have enough of them look through your inventory for Coin Chests. When you open them, you have a chance of earning several kinds of Coins that can be traded for Pirate Coins. It is as simple as talking to Traveling Merchants throughout ports. If not, you can utilize the usual methods of getting more, such as quests and daily shipments.
Once you're there head to the south towards on the Deck Ship and talk to the Treasure Hunter Igran. The Treasure Hunter Igran will offer you items in exchange for Pirate coins, including songs like the Song of Resonance.
It will cost you 16,500 Pirate coins, so you'll likely 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. Opening those gives you an opportunity to earn several types of Coins that can be traded into Pirate Coins. Simply talk to Traveling merchants who are dotted in ports. In Buy Lost Ark Power leveling other cases, you can make use of the usual methods of getting more, like quests and daily shipments.