That is a jet. Do you really want it to be a UFO. Maybe it is a UFO since we can't indentify that exact type of jet it was...
I bet if they listened closely they could hear the roar of the engines or the sonic Boom. I did not see and clouds though. Clouds usually make the air more dense, which allows the sound to be heard better and louder. I was watching a jst fly over my work and since it has been stormy and cloudy, that jet was LOUD I was probly about the same distance away as was displayed ion the video.