I noticed that the people in the crowd aren’t even really watching or looking at what is happening to the towers. People are just walking along or talking idly with no real fear or worry

Notice too the camera is filming the crowd basically from the nose and eyes upwards, so you can’t really see who is talking and saying what. Meanwhile the same ‘oh my god’ is repeated in the exact same tone several times as well as several other phrases and words from the crowd. It sounds like overdubbed crowd noise to me.

The video seems made of three layers: the towers in the distance; a crowd of people watching something (which now are repurposed to ‘watch’ the towers); and the two or three people closest to the person filming and which you do see actually speak some of the words you hear. The rest of the words seem dubbed in.