[mnehring]

It all depends on how fast it was flying and over what terrain. If going faster than sound over an area that would absorb the sound, it is possible.

[svcw]

I do not think your analysis of sound is correct but even if it is, the craft appeared to be stationary (hovering).

[Quix]

It’s also quite possible that the craft generated no sound to be heard, even at close range. We have such technologies.