Thank you for playing!
The physics do change between levels, but I could tweak it to make the change a little more extreme. I was worried that if it was too drastic, it would only be disorienting.
I have to double check, but I think that bug may be due to the initial launch of the game not having a save file and thus the slider defaults to 0. But I'll test and see what else may be causing it!
And yeah, the shells were a currency that was meant to be used in a shop, but those mechanics were unfortunately scrapped due to not being complete in time.
But ooh! There is a collectable menu (C || Slash || Triangle) and the volume slider is accessible anywhere in the game (ESC || Backspace || Start). I made the mistake of explaining the controls in the description rather than in-game, but that's something I need to implement in the future.