Accordian Songs, also called "Notes", are a collectible item that currently only allow Reshnha to open specific locked doors. There are currently only 2 collectible songs, though it seems there may be more added in future releases as Stomidori's letter states that some songs can be used to travel to different locations in the Dream World. Cerasus' Itch.io page also states that Reshnha will need to collect songs in order to escape the Dream World.
Crimson Notes
Unlocked By:
Death and Birth of Thieves Event
Unlockable Locations
Debtor's Suicide Room, Moonbow Laundromat Back Room
Royal Notes
Unlocked By:
Although not implemented in-game, one can find the names of the rest of the songs in the common events section of the game's code. The full list of songs is Crimson, Tangerine, Xanthic, Verdant, Azure, Anil, and Royal. Curiously, these are all color based names, and all in rainbow order...
These will most likely be implemented in future updates. No audio files for these songs exist currently, so it is not possible to hear them.