If you have collected all the eggs and learned the song the eggs give you then go to the place where "The girl with no voice/lead singer of the band is and play it in front of her

There's one huge flaw with what you just said. Let me explain , to get to the fortress where you get the eggs you have to go east of the lab place and past the big metal wall . go under water and you will notice three big boards . with the zora mask on bump into one of the boards and one of them will uncover a secret passage . go in and in the fortress you will find the eggs . once you bring all the eggs to the lab they will show you a song . play the song in front of the hot chick zora who lost her voice . make sure you play the song in front of her as a zora . NOTE: if you go to far east past that big place and the metal wall you end back up at the shore . and you need ephena to jump the fence . and to get the zora mask you push the almost dead zora onto land and play the song of healing . then you will get the zora mask.