Hey there. Just saw this game and thought I would try it out since it sounded similar to my game. Had a play through it and I have a few thoughts!
First off, I like what you did with the menu. It's simplistic yet looks very clean and is easy to use. One thing I would recommend is playing a preview of the song you select in the song selection area of the menu so you know what the song sounds like.
Also if you want to avoid the game freezing when you click play while it's loading I would recommend loading in the music when the game is launched so that way the player only has to wait once.
I noticed that it's not very clear what the difficulties change, I realize you have a certain amount of lives but there is no way (that I saw at least) to tell how many you have in game so I would make sure to add a health bar or lives counter or something into the game so the player knows.
Now for in-game, I'm not sure if the blocks spawn to the rhythm but from my experience they appeared to spawn on a timer not related to the song. For a rhythm game I would not recommend this, I would use the song's BPM to calculate the beats per second (BPM / 60) and then decide how often you want to spawn blocks, once per beat is a good amount but may be a little fast, so I'd go for 1 or 2 blocks per beat.
I understand what you are going for with "Shooting" arrows towards the blocks to destroy them but I think there should be a certain required timing for this so the game can be played to the rhythm. This would make the game feel a lot more satisfying to play as you are hitting keys along with the beat as apposed to just trying to hit the right sequence as fast as you can.
That brings me to the line in the middle of the screen. Is that supposed to signify where you should hit the blocks? is it where the blocks should not reach? Is it just there for looks? It's very hard to tell but I think you should inform the player before the game starts on how to play the game, even though it is simplistic I would still put in a quick tutorial or at least message on how to play the game, avoid giving the player a huge wall of text to read through however as players often tend to skip it and have no idea how to play.
When an arrow hits a block I think the game should give you some visual feedback, such as an explosion of particles or something to make the hit feel a lot more significant, This would give the player a feeling that they are impacting on the game and makes the game a lot more satisfying. It's the little things that count sometimes, add particles to your game, I promise it will make a huge difference!
I'm not sure if it's intended, but if you don't hit any blocks and let them pass through nothing happens. Shouldn't you lose lives when they go off screen? The only way it seems you lose at the moment is hitting the wrong block with the wrong arrow.
Another point I would like to make is I don't think the music fits that well with the game, also do you have permission from the developers of those songs to use their music? Because correct me if I'm wrong but I think that some of the music is copyrighted... I would go for songs where you can feel the rhythm a lot more since it's a rhythm game.
One other thing, When you get a game over there is no lose animation or anything, the game just cuts to the game over screen. Even something as simple as the screen fading to black or the music slowing down would make losing a lot better and not just sudden.
One final note about what I think could be improved is your website here, I think you should add a larger description. Maybe add a list of things you intend to add to the game in the future, make a quick video of the game and show it here so people know what to expect when downloading your game. Maybe show a few images or even GIFS. The theme could also be made a little more colorful or just less boring.
I know that's a lot of criticism but I personally think any feedback is good feedback. I'm not trying to call your game bad or anything, I'm just trying to offer advice which I would appreciate myself if I had made this game :) For what this game is now, I would keep it up! There's a lot of improvements to make but that's part of the journey.
I wish you luck with this game and hope it goes well for you! Keep it up :D