As you can see, one size doesn't fit all, but, as Paul Burke said, it's generally (not exclusively) the tradition to quiet down for singers. At the Starry Plough sesion in Berkeley, CA., run by Irish singer Shay Black, the rule is complete quiet from all patrons when there are singers. Being a regular Sunday night session, with three short periods set aside for singers (who are called to sing by Shay) the quiet is the expected norm. Shay, though, has the respect and reputation as a singer that he can pull it off, so most of the non-musician pub goers cooperate well with the format. Newcomers pretty well follow what everyone else is doing, with the rare occasional person who doesn't like it getting up and leaving (I've seen that once in the last two years).

Tunes are another matter. On occasion there's a solo that begs for general quiet, but it's asking too much from nonmusic-loving patrons and listening patrons alike to quiet down for the tunes, as noted in other posts. Talking through tunes is par for the course, and I think most session musicians understand that pub goers come to drink and socialize as well as listen, and many are not there to listen at all.

I'd say just go with the flow with how each pub session works, and if a particular format doesn't work for you, seek out those pubs which run things more to your taste.

Chanteyranger