As long as you can transfer enough energy to raise the vapor pressure of the liquid to atmospheric pressure, it doesn't matter how quickly you do it. All you need is a method of near-instantaneous energy transfer. To wit: the microwave, or better yet the hydrogen bomb... of which this item is neither, since those transfer energy via EM waves, and not sound waves, which are physical pressure oscillations.

I doubt he's using only sound to do it. Low frequencies would probably just disturb the water a lot. The higher you go, the potential is there for a smoother energy transfer to the water (like the sonic mixers in #8 AM's link), but if high sonic frequencies cause water to boil, then every pregnant woman getting an ultrasound is boiling her baby!