definitely a hierarchy here:-

concert - total hush (you're all here to listen)
folk club - whisper (you can say it after, or next week)
singaround - maybe you won't see your friend for another 3 months, but go outside to natter (I'm often guilty on this one - sorry MBoy)
pub gig - don't expect them all to listen, some are only there to get drunk.
session - just pray that the punters aren't making so much noise that you can't hear the other players.
And try to avoid cup final nights.

And make sure your own phone's off before you cuss someone else!