Leitmotif

"A leitmotif or leitmotiv is a "short, recurring musical phrase" associated with a particular person, place, or idea. It is closely related to the musical concepts of idée fixe or motto-theme."

Mono wants to break free of this never-ending nightmare, and he learns that the key may lie in the song that he silenced during his previous cycles. There has to be a way for him to put an end to it all without having to destroy the very thing that Six finds comfort in.

[Major Spoilers to the ending of Little Nightmares 2]