For the problem of the next button, I think you are mistaking lv 4 for lv 3. I didn't have time (personally, needed to go because I was working early the next day) to make a proper end screen. (lv 1 is so short I understand the confusion). Sfx were balanced according to the copyrighted music I was using in testing that's why they are low (but yeah now it is very low as the only sound). You can try out accessing lv 4 with the select lv screen (and tell me if it is really new for you or just the lv that you keept beating).
Noted for the quit button it was good in practice but yeah in web it is not good at all
Anyways thanks a lot for the feedback <3, I was not sure for the coin placement but happy that it worked out (if I one day update the game it will probably come with special coins that you can have only in a certain gravity).