Hi Jolillo,
In principle a Kiev can be brought to the top levels of unaudibility at all speeds. In order to obtain it you need:
a) to distension the shutter up to the minimun necessary for correct speeds
b) to have all the shutter mechanism clean and lubricated, both to respectable degree
c) to use an original but tight Kiev leather low case.
Additionally I have applyied to my Kiev user cameras a bit of oil to all the spaces between the curtain links. Beware: This proceeding is not backed by any source and it is out of my imagination only. I embarked in this process out of my needs of a truly silent rangefinder camera.
You should apply the oil with the help of a cotton tip, in an horizontal movement trying to aim at the spaces beteween the curtain links.
Of course we don't want that when the camera is loaded with film the oiled shutter curtains will splash oil to the film. Therefore, after you are done with the oil aplication, and after some shutter firing, you sould start a process of cleaning the curtains, or absorbing the excess of oil, again with a clean cotton tip or some very small piece of absorving fabric..
However let it be crystal clear that from all the measures I mentioned here, distensioning the shutter is the most important, alongside cleaning and lubricating.
Now let me explain the connection of things.
With a mechanically dirty Kiev you can oil or grease it and have it activated, but only at the expense of a supertensioned shutter, covering for the dirt.
But I don't think you can distension a dirty Kiev, because in the process of distensioning the shutter, most of the chances are that you will distension it from, let's say, 15 turns to 0, and then re-tension it, to let's say 6,5 turns of the concerned screw. And retensioning a lot of dirty gears will not produce, in my opinion, a consistent result.
Therefore, in order to achieve the main stage in silencing your camera - distensioning the shutter - you will have to clean and lubricate it first. Cleaning and lubrication by themselves, without distensioning of the shutter, will produce a certain amount of silencing of the shutter. But relatively to the shutter distension, this amount will be marginal.
Last remark, when I refer to cleaning the gears, I mean cleaning them to death, by loupe inspection.
Cheers,
Ruben