Apparently the whole thing has been staged. What I don't get is how some of them even had official people noting the sound (such as sports commentator) which indicate the people were actually making these sounds very loudly, and not just taking a video clip and dubbing the sound in (at least those particular ones). The one at a baseball game is rather interesting. Someone would of had to been doing it in the parking lot or something without being noticed, and the sound would have to sound "natural" and not like it's coming from one direction.
But it seems it has been confirmed it's just a hoax. I kinda had a feeling, but was hoping it was something more fun.
My first thought was that it was the US military testing some kind of new aircraft that did not really have stealth in mind, but maybe speed thus very loud powerful engines.