Piers & niosis, I can't believe you made this in such a short time span!
Not only are the visuals and sound incredibly unique and polished, the rhythm gameplay is also super addicting. I thought the worldbuilding and narrative framing of the game were super interesting as well, and the fact that this runs as well as it does on web is a feat of optimization.
In terms of feedback, I'd say that if this were a longer game I'd love to see some combat variety. Maybe there are combos or weapons you can unlock that have various benefits, or enemies who change the song on you, like speeding or slowing the tempo. I would also love to see your worldbuilding expanded on in small cutscenes or dialogue, perhaps in a hub world environment between levels.
For a game jam though, I think this game is perfect. Wonderful work!