When I got my first mechanical keyboard, I opted for Cherry brown switches because they were quieter than blue switches and at the time (2010) these were the only two switches that I could get locally.
They were too loud, though. My girlfriend complained when we shared an office. I tried o-rings but it made the switches feel much worse.
I tried the QMX switch silencers and a mat, but it doesn't sufficiently make it quiet and it also feels bad.
I ended up buying a HHKB that came lubed and had Hypersphere rings installed. The keyboard ended up being quieter than an Apple chicklet keyboard and it was great at home and at work.
I've since bought other keyboards but I always go as far as I can to make my keyboards quieter.
For Topre:
- Silence the sliders with either purple sliders or silencing rings like Silent-X/Deskey
- Lube the stabs - some Topre boards have rattly stabs like my FC980C.
For MX:
- Silent switches if I can
- Lube switches
- Replace the stabs and/or lube them. Stock stabs on off-the-shelf keyboards will give you less options for swapping but lubing can at least take away some of the rattling.
I also put a layer of foam/neoprene or something similar inside the case to absorb a bit of the noise. It might not necessarily make it quieter, but it removes some of the higher pitch noises.
Using a desk-mat underneath can help too, which I always do for aesthetics and for having a mouse pad.