These events are not available every day. They usually take place on weekends. You can also participate twice in the activities of the island during the weekend.
There are three event quests in the island. For each event you will receive a chest: Sparkling Sound Chest, Aromatic Sound Chest and Clear Sound Chest.
In these chests you will have a chance to get this song. It is possible that you don’t get the first time but you have to try your luck again the next time the event appears.
You can also do the island events to get other chests in hopes of getting the Heavenly Harmony song.