I personally use MX Browns at home, but they're not significantly louder than a full size, well used membrane keyboard. Membrane keyboards tend to get noisier as they age and their stems wear down and their lubricants degrade.

On the other hand, there are far more silent mechanical switches available. There are "Silent Brown" switches, "Red" switches without any tactile feedback, or "Silent Red" switches, with even less noise.

If I decide to change my office keyboard one day, I'd buy Red or Silent Brown switches, not because standard Browns are actually annoyingly noisy, but we as people are under stress in the office and get irritated sometimes, and I want to be polite.

As I said before, mechanical keyboards are not about the noise they make primarily, they're much more comfortable to use and they last a long time. Oh, I like the sound of my Browns, but it's just a secondary effect.