Same here last Wednesday at an Arcade Fire gig...the couple to my left were nattering loudly the whole time, getting louder whenever the music got louder.

After 4 songs I turned to them and said "I don't know why you bothered coming here, you could have sat at home and chatted with the albums on as background".

That shut them up for about 10 minutes, then they kicked off again. After the next song I turned to them and said, very loudly: "my daughter bought me a ticket to see Arcade Fire as a birthday present. She's a student and it cost her a big part of her disposable income for the month. You can shut the fuck up now or you can refund her the ticket price because I can't hear the bloody music due to your constant talking".

That seemed to do the trick...or maybe it was the 6 people in the seats behind who applauded my outburst, but they were pretty quiet for the rest of the gig.