Hi! I was the artist on the game, thanks for the review!
More mechanics would be awesome, I like the idea of spikes as we have already implemented them visually. We tried to make it easy on ourselves and focused on the simple mechanics we had, but I agree that we could have done more. Regarding the sounds, honestly we totally forgot about the sound requirements and we also had someone who made awesome tracks so we didn't think twice. I also think checkpoints could have made the game a bit more bearable, because it does get pretty hard once the game mode switches! Instead, we implemented lives. We had some ideas for checkpoints but in the end didn't go through with them and stuck with the lives, but it's never too late to implement them in an update!