Well, if you have the latest patch the notes should light up as you sing them in the right order. Notice the carvings and murals on the walls? You can click on them. They give you a clue about how to use the song.
Whoops, thought you were in the SONG cave. Well, if you've realized that you can draw the fish to you by singing the right notes...have you looked around the cave for any familiar symbols? It's not a song you're singing.