Season 2 Ep.7. This is the Casino run by Tribe Circus as KC and Lex found it in Season 1. And this is the same Casino in Season 2.
It's the same Casino, or supposed to be, because the doormen recognise Lex and appologize profusely. Last time (Season 1) they beat him up even though he had the Virus. Now he's "Lex of the Mallrats". And Mallrats are the respected and honored Tribe that gives everyone free Antidote. They invite him inside for a game of Roulette.
As they open the door, we can hear the noise from everyone inside. But there isn't anyone. It's empty. So who's making all that background ambiance? This is where the sounds come from. And while Lex is playing we see brief footage showing a casino full of people gambling. But it's footage from Season 1 when KC and Cloe were here - or there - because it really can't be the same place, inside or outside.