Tried to start the first song and just got a black screen. Refreshed the page and it looked like that game was gone so I just started the next one instead.
Menues being mouse-only while gameplay uses keyboard means players constantly have to switch back and forth. Why not have the menus navigable with the keyboard?
The UI voice is a little annoying, I'd prefer a more standard sound effect.
I didn't see much of a difficulty curve, prompts come from all directions immediately. I think it'd ease more into gameplay to have arrows start from only one direction, just to teach the players the pattern. Then entirely from the opposite direction. Then alternating between each direction for full sections. Then mixing those two directions at the same time. Give them a chance to master that before even adding a third direction, or the forth. Let them ease into the challenge instead of being bombarded in the first moments.
Aside from that, the prompts should match with the music. There seemed to be no prompt/note correlation, making the music more of a distraction that a feature.