Thanks for the feedback!
I would like to explain some reasonings behind of the stuff you pointed out.
Making the enemies predefined will be quite laggy aa I have tried doing something of that sort . I know the current spawning is not very good as it indeed has the issues you pointed out. I will be fixing them for the next update.
I have been thinking of making a xp indicator, but as I thought it wouldn't be needed I didn't implement it. The level up interaction will be reworked in the next update, as it will focus mostly on that.
Currently the sounds have problems, but there hasn't been this inconsistency, which you mentioned - the music being silent while the effects deafening. I will look into it anyway.
Anyways, I hope you enjoyed this version and I very much appreciate your comment!