I agree with @xyz , timer is pretty useless and frustrating and sounds are very needed.
Also, WASD are mostly good for EN keyboards, arrow keys would be at least welcome and later key redefinition in settings.
It could be also good to add a little info at the end of the demo telling more levels will eventually come later, not a non reacting retry/next pair of buttons.
So good job so far still lot's of work remain.
By the way, I've started a project I called "Game Foundation" with the purpose of providing a main menu and everything needed to make the boring part of game creation easier with the ultimate goal to just "plug in" the game into it, fine tune stuff and voilĂ , have a game with professional look, with all options like profile, settings (for 2D/3D/whatever), network options, cloud saves, languages. Saving hundreds (literally) of hours of coding & designing UI.
Regarding the sound part, I've come up with this in my Ishid-O-Matic Reborn game by creating a top-level/root scene which manage all sounds & musics, and the other parts just need to deal with signals and proper global data to know which sound to play in which case.
It's still the very beginning still it can be interesting to some people here I guess.