Overall I really like the game feel and execution of the ideas. Very similar to what I was going for in my game (although this one is much more Stanly Parable, Superliminal, Portal, whereas mine was more like Half-life + Terraria had a Portal baby). Great job!
Misc Notes:
That audio buzz at the start might be too realistic, I thought the game was crashing on launch. might want to reconsider having that on the main menu on startup...
jumping is a bit floaty, might want to scrap gravity based jumping for a velocity curve jump if you are going to have the player doing anything more than basic platforming.
Getting the enemies to die is a bit clunky and frustrating (if they are supposed to take 5 cubes to the face to kill, at least show that in a health bar!).
Also the cubes spawning and shooting out off-center to the camera makes aiming hard, if you want the player to be shooting cubes accurately as an attack you might want to change that, or just make the ragdoll kills easier to achieve with less accuracy.
The enemies are super creepy and creative. I love their aesthetic and animation and sounds. Great design all-around;
being able to store cubes after shooting them would be cool. without adding that one might be tempted to conserve ammo by running around picking up the same cube which would get tedious.
Maybe even scrap the ammo system entirely, make cubes disintegrate over time or even better after a certain number are in the scene and let the player shoot as many as they want with a cooldown. This might get weird with using those same cubes for puzzles, but it also might be a really cool solution to the player just using 30 cubes of ammo to solve a puzzle vs. them knowing they can only use like 3-5 cubes before they start dissolving the older one. This has the added benefit of once a puzzle is completed, they can just move on to the next section and start shooting cubes to use for the new one without having to run around collecting cubes or whatever. This would allow you to have longer form levels with less elevator intermissions.
The enemies need more audio cues so you don't just enter a room and die immediately when they are right there. It can also be annoying when focusing on enemies in front of you and one gets you from behind completely silently.
The audio for the enemies reminds me of Alan Wake 2 shadow people which is neat. This game was more entertaining than that one though :P.