The Tunnel of the National Institute of Notalium is the setting of Chapter 3.
Basically, it is meant to be a way to move to many different locations. Known locations include: a city in the south, the Silence Border Guard, Presto (a lab), Allegro (a lab), the Farm Village, and another new location that the story will reveal in Chapter 4 once it's released.
The abnormally large instruments present appear to have all been made by one person because there is a passage of text that appears after completing three challenges for each song. All of which seem to come from a single poem, which can be found here, but on You are the Miserable, the text only reads, "Wait for me, Ena." It is unknown as to who made the instruments.
Because of being adjoined with the instruments, the Notalium in the tunnel happens to sound energy based heavily on the instrument it's attached to, Song for Sprites: Woodwind Instruments, Phoebus: Brass Instruments, Duelo: String Instruments, Androgynos A and B: Percussion Instruments, and You are the Miserable: Pipe Organ.