Thank you for playing and for your comment!
The music only stops in the web version, I’m not really sure why yet, but I’ll look into it.
As for level 4, that’s where the game is most likely to crash because it’s very easy to die multiple times in a short period, especially near the spawn area. The issue was simply that if you died twice consecutively too fast, the black transition between deaths didn’t have enough time to finish.
I’ll be adding SHIFT and SPACE as buttons to change gravity in the next update.
Thanks again for your feedback!