Sound room humidity

It probably varies to some extent with climate.  56% is my number in South Florida.  This is semi-tropical.  40% would not be practical here.  The air is thick here in more ways than one.  Consider the musings of our President and Mr. DeSantis.  ;-)

No. Humidity is not really an issue unless it becomes extreme. Here in Florida with the house always closed and the AC system always set to dehumidify we are lucky to get under 60% RH. I see the same concern on camera forums where people are buying dry cabinets just because they bought a new camera and are new to the hobby. Same for stereo stuff. I'm still using gear from the 1980s as are most of my friends who are into audio. Just not an issue unless you are above about 75% regularly and that also becomes an issue for structures :-) 

Personally I find 55% feels too humid.  I'd keep it at 50% here year round if I could.  :) 

It depends on the temperature.  It's called "relative" humidity.  A 50% relative humidity at 80 degrees contains much more moisture than 50% relative humidity at 65 degrees.  The higher the temperature, the more moisture it can contain.  Dewpoint is a better measure, particularly when determing what it "feels like".  As to how this applies to audio, I have no opinion/knowledge.

I watched an interesting video on Rick Beato’s site, he was interviewing a sound guy who did live concert sound. He mentioned that they must take humidity into consideration when setting up the sound in the live event as it effects the sound quite a bit. He gave no humidity levels, but it makes sense.