This one rings a bell (looking back to the original post).
We had a similar incident at a singaround a while back, where an elderly couple talked loudly (intermittently) during songs. It was very unfortunate that the lady tended to be commenting on the songs & the people there. For example 'Oooh - I'm not sure if that one's male or female', and 'Well, its all quite pleasant - but none of them's really good are they?'. They were sat right behind me, so I had the benefit of most of these insights - and I'm pretty sure the voice carried throughout the room.
Although it was very funny, we were starting to be a little fed up with the couple - we'd tried some subtle shushing &tc to no avail. We didn't feel it was right to go any further as it is a pub & they were there before the music started anyway. They were invited several times to sing & seemed tempted but declined (she joined in with some bits & pieces that were familiar though)
Eventually they got up to leave (during a song of course) - and they said goodbye to everyone, thanked us for the songs & said how much they had enjoyed it and enquired was it on every week. They obviously had no idea that they had breached the folky ettiquette - and to be honest there's no reason they should know our sensitivities. There are after all lots of occasions when its perfectly OK to talk over music & singing & maybe they were used to those kind of events. Also - some people have no idea how loud their voices are. We all bid them a fond farewell, and reminded them that we'd love them to sing something next time please.
I was so pleased that we hadn't got into an argument, because they turned out to be such a nice pair and had clearly intended no malice. I think we all had a warm feeling (ahhhh) and saw the funny side.
Perhaps Roger's drunken lady will turn up next time & be totally different (perhaps it had been a bad day...). We haven't seen the couple since, but I hope that they feel welcome to come again any time & I expect that if they came again they would be likely to pick up the expected behaviour & would feel part of the group enough that a word in the ear would be sufficient. I think that Karen's idea about being part of the group hits the nail on the head.
Another trick I've seen done in a generally noisy pub environment is for someone very able & confident (ie an experienced performer) to take a turn & sing something very quiet. The chatterers will often either listen or start to talk very quietly as a reflex action. Has to be someone charismatic & authoritative though. The volume takes a while to creep back up.
Unfortunately I don't have the balls or skill to do that, so I just sing really really loud - which of course has the opposite effect as people have to talk louder to be heard. Tragically that's my reflex response - and it just makes things worse.......... Oh well - I'll try to be a better person in future..

Kris