I played all the levels... which means it's good! +Music, +Sfx, +even more gameplay elements and you are in good shape
Maybe add time pressure? Even more mechanics? It will begin to feel like game "N". Level editor in the future as well?
Or maybe some other mechanic that will change it alltogether to make it unique?
Anyway, it's good even as it is now, just add sfx&music Congrats.