Could a man hold sustained flight using sound?
Hmm, I'd never thought about. There might be some clever way to do it, but I sure can't think of one. Sound waves are pressure waves. So you get a pressure that moves an object in some direction, but a millisecond or so later you get rarification (reduced pressure) that moves the object in the opposite direction, so it cancels out. If you were moving at the speed of sound then you could "surf" a pressure wave, but if you yourself are producing the sound then it will never catch up to speed-of-sound you.
That levitation seems to be like this. You have a standing wave, so there are static regions of pressure that are more dense than the object being levitated. Areas of air that are more dense than a human body are not going to happen.
If there is a lot of thrust coming out of his mouth, why doesn't it blow his head off? Even if it didn't, why doesn't he tumble end over end?
You might have more luck having this sound reduce something's mass. Uh, maybe the frequency is so high it disturbs the strong force in something's atoms. But that would be a lot more likely to cause some ungodly explosion.
I don't see any way to do it.