I designed Creality K1/K1C bottom and back fan covers that should silence the fans without minimising air back preasure. To use them and make effective aditional actions is needed. You need to cut out grill, change fan or add voltage regulator/stepdown converter.
Read the whole tutorial to understand the whole concept how to minimise K1's noise

The main idea was to LOWER THE NOISE of K1C fans without sacrificing the performance. So what I did:

a) hotend fan - just swapped stock fan to balanced Sunon 5015 MF50152VX- and printed also new cover in PC filament. Found this mod on https://www.printables.com/model/797223-creality-k1-silent-toolhead-covers-5015-or-5020-fa

b) power supply fan - I cut out the metal bottom plate grill near the power supply. This grill generated noise and was stopping the air flow. I printed shroud/cover with intake from he back side. Made it as tall as it stopped generating back preasure. As overall air flow became better I lowered fan speed to 70% also by adding voltage regulator. After powering on the printer, the fan is barely noticeable.

c) motherboard fan - the same grill cut and shroud. Fan is PWM so I left fan speed untouched. Whining is gone, sound level 50% less.

d) back fan - carbon filter is useless in my opinion. Filter makes hudge air back preassure. It stops half of the wind. So left only the cover from the filter... I cut out grill in the back of the printer also. Swapped original fan to Sunon KDE2406PV2. It has similar speed/preassure. I added silicone dampers from FPV drone FC too. THIS MOD ALLOWS to print PLA without opening TOP LID !!!! Printing PLA 230C with bed 50C chamber is about 37C inside with 30% fan speed (room temp 27C) - sound level ~80% OFF !!!! So on 50%... would be even better. In my opinion vents in LID IS NOT NEEDED anymore... and the closed LID also stops the noise. I mean that sound of back fan is so low that it can be used to cool chamber instead of lifting the LID.
Only part fan (side/aux) is last to be modified. I think for my mod i willuse as a base https://www.thingiverse.com/thing:6706563/ Fan shroud will take air only from outside through the hole in the side panel. My bigest concern is why printer designers made air intake inside the chamber? Any intention ? Room air always is cooler and fan coud run not at 80% speed but less. So shroud and lower speed should give more silence without any loss.
What I can say - even without this last part/aux fan mod - at this points motors started to disturb me more than fans 😃 So result is good. I feel significant noise reducion see photo (without side fan on).

I have also idea how to silence motors and overall noise - by making ANC (active noise cancelation) device. It would use the MIC inside the printer and speaker outside for active noise cancelation (or vibration speaker that could be attached to the plastic panel or lid. I found the schematic there: https://www.youtube.com/watch?v=rFGm0SStAyA&t=304s But experiments is needed ...