I've really enjoyed First Land! Between the visuals, ambient sounds and music, it's been an experience. However, once I got to the Subsurface, I got extremely stuck.
I found the worm at the end of the temple area that sings a symbol, another that opens a portal to the save crystal and the creature in the dark behind the door, and the one among the domes that seems to want a sacrifice.
I also found the IS whistle, but have yet to find a use for it, but have tried it in as many places as I can find. Some hours in, I discovered from the comments here that there was a speak function, but I still haven't found that it solves anything, despite restarting the game and taking pictures of the four phrases found in the memory sequences and using all of them everywhere. And I've found the secret area just past the mural room with the blocked door at the end of the hallway. I've also gotten I think three or four memory items, but used all except for one memory of silence. I'm at a complete loss for what to do.