Absolutely fantastic game! I've played quite a few of the ones submitted so far and this one is definitely my favorite.
The visual style is very unique and appealing and the darkness of it goes very well with the theme of "echo location". A lot of other participants used the echo location mechanic but I think that you definitely have the most unique take on it. I could definitely find myself picking this game up again at some point just to mess around with getting better times and creating chaotic bullet patterns to avoid.
If I were to offer up some ways to improve it I think the lack of cooldown on the shotgun makes it very overpowered in certain cases. I definitely think that if there is a limit it should be lenient to add to the general chaos of the game but it's just a little too lenient in it's current state. When you approach large open rooms I find it's very simple to just spam the shotgun button, exit the room, and wait for the enemies to clear. Also for the visuals, the text on the buttons and the cursor are a little difficult to see since everything is the same color, I recommend giving the buttons a black background to make the text stand out more and possibly changing the cursor color at least for the title screen.
But nonetheless amazing work, it's very clear that you are a wonderful and creative game developer!