Really fun to play, IMO you nailed the correct amount of difficulty for the bullet hell patterns and the walls closing in is a very cool mechanic! I wish there was a Continue/Retry option because I died quite a few times in the later levels and I was really curious to know if there is an ending or not.
Sounds were indeed a bit quiet. The controls feel good, I like that the levels are random, and it's very easy to pick up and play. I can definitely see this concept being expanded (no pun intended). Well done!