The sound is fake.
It it was real, the same *bang* would be on more than one, maybe all video cameras in use on the morning of 9/11.
This YouTube comments show that this video was shot north of WTC, possibly as far away as
Murray & Broadway, a thousand ft or more away from the nearest WTC building. The comments on the various YouTube videos give this as the location and show that it was filmed by one Lucia Davis, who was shooting elsewhere in the immediate vicinity. That was my stomping grounds, The corner shown in the video is consistent with the corner and the area.
If that explosion was as represented and that loud, it would have been heard by everyone below Canal St. It wasn't. That would have been maybe a million people.
This comments in this version of the video shows that it was filmed by Lucia Davis.
http://www.youtube.com/watch?v=tAoWDcu9r8A
Davis is in IMDB. I'd love to find someone with a "pro" subscription and contact her and ask if she as heard that she is being put on YouTube, rarely with credit and possibly hacked.
http://www.imdb.com/name/nm0205059/
I believe the YouTube video is clipped (without credit) from film. I haven't been able to view it but I bet the sounds are not on it and that a larger context will show that the instant the guy was on the phone was unrelated to any collapse.
IN MEMORIAM: NEW YORK CITY, 9/11/01
http://www.hbo.com/docs/911/credits.html
Davis is mentioned in this tangential article I found when I was
trying to contact her.
http://www.nordicom.gu.se/common/publ_pdf/32_117-126.pdf