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?"

I have a theory that's based on modern music technology versus general human interaction. In our world we run a radio or have the TV on and manage to talk while it blares in the background. However, we generally shut up and listen when someone is talking to us. When a musician or group of musicians are playing instruments and singing the mental picture is similar to running that radio. How many people shut up and listen to the rock band playing in the bar? What they do is either dance or try to yell into each other's ears.

But when a person stands alone on stage, singing unaccompanied, then the mental picture is similar to someone talking directly to you. And you cannot talk without interrupting them.

At least that's my theory.