I made it to the first inverted level on a timer, with the big machine guns. I fell, again. Then I couldn't go back there and ended up rage quitting, because I've spent too much time on it.
One major piece of advice: push players to the shop
The first time I did the first level with turrets, I solved it accidentally, and then I couldn't figure out how, and almost quit the game altogether. I later realized that I had solved it by jumping while in the air, which is also very much non-obvious. That's a really neat mechanic though, so you should probably add some text explaining it. Even during the jam's voting period, you're free to edit the itch page, so add something there that explains how to play in a less frustrating way! It's a great game and I'm afraid you'll turn away many players who won't understand how to progress.
Gameplay
Perfect. The controls feel smooth, even with the orange cube it feels super natural to control it (I usually struggle with fast-paced characters) and I was able to make precise jumps pretty easily. Only issue was sometimes after a high jump it feels like you get stuck in the ground, and this can mess up some jumps.
Graphics
The art is really neat, but more feedback on some elements would be appreciated, notably mines (I initially thought they were buttons and didn't understand why pressing a button would restart the game). Camera movement could also use some work in the larger levels, maybe move it in the direction of the player's movement on the X axis so we can see further, and increase its drag on the Y axis so jumping with 4th cube doesn't get nauseous :p
Audio
Again really good, as others mentioned the soundtrack restarting on death is quite annoying at the start. Some feedback would also be appreciated on damage / coins / level finish, especially when the key disappears and nothing happens and you wonder if it's bugged - maybe a popping sound from wherever the key spawned?
Innovation
Platformers aren't new. But this one is particularly vicious, and I kinda like that, so you get 4 stars even if I hated loving it :)
Theme interpretation
I feel like you've added "Bounds" in the title just to earn my 5th star. It worked.