Personally I draw distinctions between different settings. For open mikes (especially in pubs) I expect a fair bit of noise, since a lot of people will be there for other reasons; In pub sing-arounds I expect some noise, but less than in an open mike; And in a sing-around at, say, a folk festival, I expect quiet during the singing/playing. Plenty of exceptions, of course, and no matter the venue there seems to be one loud drunk at every second or third session!