Great demo! The max volume in the GBA version is pretty low in comparison to the original cart games, I can barely hear any music when playing on the console (the first model) in the environments where there is even a little background noise...
I feel that some elements that the player can interact with, e.g. the platforms that you can jump onto could be more visible in comparison to other background elements? Maybe experiment more with shading / coloring, to make some objects more "pronounced"?
Regards