In this past week's episode (May 11th) there was a hand where someone went all in (I think Gabe Kaplan) and then joked after the hand how he could hear Mike Sexton at the other end of the room saying that the hand was for all his chips and how if he lost how he'd be out of the tournament.

The cameras then cut to Mike and Vince sort of laughing and acknowledging Gabe waving to them. This got me wondering...if Gabe could overhear this, how is it that the announcers can safely say what the hole cards are? Couldn't this be overheard as well? I just figured that Mike and Vince went back and did the announcing afterwards from a taped version of the match to protect the secrecy of the cards. (or if they announced the match live, they'd at least be in some sound proof structure).

no

They mainly just do the all in/showdown commentary

They watch it later and record the commentary for the hole cards so the other players dont hear

They are doing just the basic "blind" commentary while at the event. Then they go back tot he studio and edit out the crappy hands, and then have Mike and Vince do the voiceover commentary when they can see the hole cards.

That's why it takes about 9 months to air the episode after the event.