I've been known to repeat the last sentence for them when one audience member couldn't hear the other over my singing. It usually surprises them. I know that it's probably rude of them in a concert setting but most of the time, I think they just don't realize how loud they are. I think musicians are the worst but I'd put that down to nervous energy.
As for Pubs and such: We're paid to entertain. If they're having a good time talking, your work is done. Just focus on the rest of them.