Thanks for the feedback! I actually wanted to add some background music/sound effects to the game, but a js error didn't allow me to. Basically, because i've used html5 to make this game (not unity or something), i couldn't play a sound when the user didn't interact with the website (eg. clicked something). That's why i could add a sound when you press "enter", but not when the ball destroys the bricks or something.
And yeah, i should definitely add a more challenging difficulty.