Editing is certainly a possibility. I truthfully don't remember how often contestants would ask for bids to be repeated during my final years there.
However, they also have installed speakers behind Contestants' Row. I'm not sure which audio feeds are being sent to those speakers (it's possible that they're exclusively for audio from Drew and/or George), but if the other Contestants' Row microphones are going there, that may also help cut down on the number of repeat requests.
I've attached two pictures. The first one was taken in June 2009 and shows the type of speakers that used to be installed back there. The second one was taken in March 2013 to document the temporary step that was placed behind Contestants' Row for the first kids' episode (so the children would be able to see over the displays), but it also shows how the speakers changed.