Return to Headstone Island (C7). Before entering the island’s cave, Red will tell Medli to do everything Link instructs. The Earth Temple is full of all sorts of unnamed dangers. The only way Link and Medli can get through it is by working together.
Afterward, head back to the stone monument in the cave and conduct the “Earth God’s Lyric” in front of the stone tablet. Medli plays along, and the tablet shatters, revealing the entrance to the Earth Temple.