Thanks @MrBundles !
Shmank#3021 did great job as sound designer, when implementing it, the game was much more immersive.
For the dark world switch effect, we used sprite mask, as you can found here : https://gitlab.com/Quantum_Martin/sprite-mask. Then we used the collision mask to make the player collide with multiple layers.
We didn't manage in time to produce multiple level. When you take the key, we detect the end of the level by "closing the door" but there are no animations here.
The week was fast ! But was a good first jam, as we learn a lot ! Hope we go further next time !