Here are some criticisms, however I do recognize that doing a metroidvania in a game jam is in itself quite a feat indeed!
- Platforming and player movement is key in a metroidvania. Dashing and double jumping doesn't work half of the time - its based on the beat, but its not clearly communicated what is the window of opportunity though. Would be good to give players a little more leeway here, and rather than make it not work, maybe make it work less effectively? Wall jumping generally isn't very reliable (its just a mashing of buttons for me) and doesn't seem to require following the beat.
- The camera feels a little jerky (zooming out when falling a distance) - maybe limiting the fall speed and moving the camera in tandem would work better.
- Didn't like the general feel of the smoke areas which prevent you from jumping - its not obvious why smoke prevents you from jumping, and you just slow down the player movement. If it was presented as a sticky goop i'd be sold on the idea though.
- Its not immediately obvious what harms you or benefits you from the minimalistic presentation, only until halfway through. Some hurt you, some bounce you back. Maybe color code it? At least the enemies need to look threatening enough.
- General rule of thumb in a metroidvania-like game in my view: to teach players new skills in a safe space where they cannot die. There were some areas where the enemies just overwhelmed me before I learnt stuff. You did a great job of guiding players to learn new skills though, and glad that dying and respawning isn't too much of a deal.
- Saxophone was a little counterintuitive - being spread over 2 powerups?
- Suggest to let the player kill stuff - as it didn't really give me the power trip that usually comes with a metroidvania (where you grow stronger over time). Or maybe I didn't complete it all the way through? Got the moustache and reached the signboard but didn't find a way into the area with a bunch of collectibles.
- The minimalistic style makes the exploration feel rather monotonous, although understandable for a game jam. I loved the boulder segment though! You could have a look at the game 140 for ideas on how to make a minimalistic music game look really good.
That said, the idea of unlocking new layers of music to cause level changes is a really cool idea for a metroidvania. Also, I liked the platforms moving to the beat, they remind me of the music rooms in Fez. All in all, I did have fun!