Naeamonson, you said "I don't think anyone has addressed GerMan's original question, "Why do people at sessions shut up & listen to a singer but when someone with, for example, a guitar does something everybody starts/carries on talking?"
(A-hem!) I did address that question a couple of posts above yours. Not a definitive answer, of course, but one plausible one, I think.
"As for talking over instrumentals and not over vocals, it's simple. You can hear and appreciate music without focusing on it with your left brain. But you won't process lyrics while you're chatting or otherwise distracted."

Genie

PS,
I usually call it "Acapulco" singing, instead of "any comp kneed."