Thank you so much for the effort you put in for this review. Really didn't expect someone to play my game for almost half an hour. Your commentary was spot on and I'm grateful for your suggestions and advice, I actually took notes when watching.
I completely forgot to confine the cursor to the screen despite knowing about that feature of Unity, since I always playtested in fullscreen on a single monitor.
As for the parallax effect, the further background is actually a skybox, so that's why it's static. I've never actually done parallax before, so I wasn't sure how to go about it and decided to just make a skybox and then put a semi-transparent plane at some distance behind the action. I'm going to try your advice on how to make it look less like dust on the screen in a future version.
Now to answer your questions. The soundtrack consists of 4 tracks, which are picked at random, regardless of speed or time left. Whenever one of them finishes playing, another starts (though due to the difficulty, it's rare to actually hear this happen, and since you did, you're now officially gud™ at this game :) ). The key of the no-energy sound matching the music is a complete coincidence.
As for enemies, several of them are pre-placed (that's why the game starts in the same way every time), then, every 20 seconds a bunch of enemies of both types is spawned at some distance outside of the circular border and they fly into the arena to get you. So, it isn't actually safer near the border. The spawning mechanism is a bit random and can be problematic: it consists of a number of spawn points around the arena, each of which has a small probability of spawning one of the two enemies every 20 seconds. So, it can be very unfair if too many enemies spawn near the player (in which case the player will be horribly murdered) or not enough (in which case the player will die from timeout).
Thank you once again for your video. I've uploaded a postjam build, which fixes the bugs with highscore and time being counted even while the game is paused. It also features difficulty selection (though it simply affects the max number of bullets and the time given), confines the cursor to the window and incorporates your suggestion to make the smaller enemies' hitboxes a bit bigger. Do check it out, if you'd like.
The player still looks like a hair dryer though. :D