I liked the underwater/hell parts I think you can add a lot of challenge to them just by adding some moving parts to the levels so the player can't take their time sweet time! Maybe even using wasd for the controls while keeping the way the player's movement works could make it feel nicer to control. jumping is one of the player's few actions so maybe adding bark sound effect and a different animation for that would make it feel better. For a jump animation you could just use a still frame of the walk animation where the legs are out the most.
well since I'm pretty new to the flowlab game engine everything was more complex so if I wanted to play a new song I had to assign a different block a property on each level with a new song and to remove the song on the next level. I would love to add moving bits but I sadly don't know how. but the idea with a barking sound effect could work I would just make it happen on some of the jumps so it wont ruin the experience by playing a sound effect that covers the music. lastly I will try to add a better jump animation after the jam.