About last level again. It's ok if it's difficult, since previous levels already ease you in the game mechanics. You know how everything works, so you are ready to take on the challenge. Also- I think you should lean in into those vibrant colors. Check this. This is 'Spent Casings'- one of the other games from this jam (worth checking, I might add). It looks cool, because it went really hard on the post-processing. Something simmilar happens on your level 6. Colors are vibrant, the revolver is just a silhouette and the vision range is reduced, but it just looks cool!
As for sounds. Having any audio feedback for a gunshot is better than no sound at all. I often use freesound.org - they have a ton of sounds available with CC0 license (you are required to name the author in the credits).