To unlock the orchestra sites, you'll need to reach Ocean Rank A and unlock the underwater farm. After completing the quest Resupply The Merfolk Kingdom.

Cho Oyu will bring you to the first orchestra site and tell you all about it. You'll need to complete each orchestra site in order to unlock new rewards

• Se Pulu (10m) Orchestra Site Location
  
  Everything you'll need:
  
  Orchestra Site

  Offering

  Quantity

  Reward

  Se Pulu (10m)

  Marinip
  Bluebell Blossom
  Twinshade Sealeaf
  Bronze Kelp Essence

  3
  3
  3
  3

  Seaweed Starter
  
  
• Dua Pulu (20m) Orchestra Site Location
  
  Everything you'll need:
  
  Orchestra Site

  Offering

  Quantity

  Reward

  Dua Pulu (20m)

  Sproutfruit
  Helixpur Blossom
  Spring Sealeaf
  Silver Kelp Essence

  5
  5
  5
  5

  Junonia Shell
  
  
• Empat Pulu (40m) Orchestra Site Location
  
  Everything you'll need:
  
  Orchestra Site

  Offering

  Quantity

  Reward

  Empat Pulu (40m)

  Pink Coralcap
  Petalia
  Red Reefbud
  Gold Kelp Essence

  5
  5
  5
  5

  Flame Scallop
  
  
• Lima Pulu (50m) Orchestra Site Location
  
  Everything you'll need:
  
  Orchestra Site

  Offering

  Quantity

  Reward

  Lima Pulu (50m)

  Goldglow
  Bluecole
  Mermoo Cheese Wheel
  Osmium Kelp Essence

  5
  5
  5
  5

  -

Once you bring all the items required for the offering, you'll need to complete an underwater music puzzle to fully restore the site. Each site has a different puzzle, but they can be solved the same way.

The site will display a pattern of colors, and you'll need to swim over the corresponding notes in order to play the ocean's song. You won't know which notes are what colors until you swim over them, so it might take some trial and error.