Well there's a couple of things I can say. First the hijackings could have been real. To get the phone calls CVR's..etc..etc. Planes landed some passengers quickly herded off into a military plane, original plane disposed of in some way. Seems far fetched, but it's possible.
But listen to this phone call of Ceecee lyles, one of the few that is actually recorded. The author of this video enhanced the audio, and the results are extremely disturbing. I've downloaded this audio myself and played around with it, but couldn't get it any better than he did. I agree with most of what the author says except for a few points. He thinks at the end it is the male voice breaking in again, I think it's just an attempt at (for lack of a better word) "weepiness" Also after the phone call is supposed to end, the author of the video, has the voices saying "it was great" "sorry" and then again "it was great" I think the voices are saying "You were great" "Howard" and than non disembodied but faint you can hear "testing...testing" gave me chills the first time I heard it.
http://www.youtube.com/watch?v=2bnPmyUUEjg