I'm going to have to agree that "something funny" is going on with that clip. I don't want to call it BS yet, as I really want it to be true, but the audio has certainly been modified from the original version. I loaded up that scene and played it in reverse and the audio is absolutely NOT as clear as what was posted. So unless the person who extracted and modified that audio can demonstrate what exactly was done to make the backmasked voices so clear, I remain totally skeptical.
For reference, THIS is the original (unmodified) audio and video played in reverse.
My notes, including the original pre-YouTube-mangled H.264 reversed video clip and just the reversed (unmodified) audio are here: http://netninja.com/files/lost/