I am on a quest for quieter tactile switches, and had some Cherry MX clears on hand. I've previously used o-rings or foam tape on the underside of the keycap, which helps somewhat, but doesn't do much to quiet the *release* of the key. So I did something slightly different:
At first I drilled some #60 holes in the upper shell, and threaded some dental floss through (GUM Butlerweave mint waxed free sample from a dentist visit, fyi), and it made a small improvement to the sound. But then I discovered that I get even more sound dampening by wrapping the floss completely around the shell, which is nice because no drilling is required, and it's completely reversible if you decide to go back to normal.
Here are some macro shots of the final version with more lighting:
This mod is tedious to do, but the nice thing is that it quiets both the downstroke and upstroke, and because of the "padding" you end up with on top, provides some o-ring-like cushioning on the downstroke. Also nice is that you can vary the number of turns for more or less sound dampening. I used 4 wraps on this last one and am happy with it.
To my ears, the sound is reduced by about half. There is less treble to it ("thock" instead of "snick"). Note that this is the only variety of mechanical keyswitch I have any experience with, so I can't really compare it to other MX colors, nor to things like Matias, Alps, ML, etc.
