If they don't turn off the sound on the TV, the session is over. Or if it's something like a football match on the box, the session hasn't started yet.

If they won't turn the sound off it means they don't want you, so find another place to play. Playing over the sound just means no one's going to enjoy either the music or the programme, unless it's a another bar over in the distance. ("Bar" in England normally means "bar-room", though the American meaning where it means the premises as such is creeping in in wine-bars and such. But you wouldn't find sessions in many wine-bars.

I remember when they had that famous interview with Princess Diana, the one where she made her "Queen of Hearts" bid, we were -laying and there was a television screen on, all the way through, but with the sound turned down. I remember noticing it and thinking that this showed a proper sense of priorities by the publican.