The frog's song of soul is one of the three melodies you can obtain for your ocarina in Link's Awakening. You can learn this after paying a large sum of rupees to listen to Mamu's frog choir. After this it immediately becomes usable.
Its main purpose is to revive the Flying Rooster that was but a mere skeleton under the Mabe Village weathervane before you play the song to it. It was also used to open the entrance to Turtle Rock.