1, A pure note consist of a single frequency. It's not really meaningfull or useful to talk about the frequency of a non-repeating sound - like a human voice.
2, A good way of looking at complex sounds is to split them up into a lot of single frequencies that are added together. A square wave consists of a single square frequency but you can also approximate it by a number of different frequency sin waves. A sqaure wave is a very good example of this because an ideal square wave goes from low to high in zero time at the edge, this is impossible in reality - the edges of a real world square wave will always be a little bit sloped. As you add more and more sin waves you can get closer and closer to a perfect square - the same thing happens for any other wave, you can always get closer and closer by adding more sin waves. (see http://www.mathworks.com/products/matlab/demos.html?file=/products/demos/shipping/matlab/xfourier.html)
3, The simple definition of Nyquist frequency makes sense for a sin wave. For a more complex wave the Nyquist limit is the frequency you would have to sample the highest frequency sin wave in the source with to reproduce the source perfectly. Remember the example of the square wave? There isn't a highest frequency to get a perfect square - you have to consider the maximum frequency sin wave you can use (or the maximum frequency you can play back) and as long as you capture that wave with the Nyquist frequency you have at least that level of accuracy.
It isn't possible to create an audio source in mid-air using the method you've described. This is because the two ultrasonic waves would create an audible source if the listener were standing at that spot, but those waves would continue to propagate in the same direction afterwards. You would need, as I point out below, some sort of medium which scattered the waves in all directions to make it seem as if the sound were coming from the point at which you interfered the two waves.
It is possible, however, to make the user percieve the sound as coming from a specific location, but it isn't as easy as the author makes it seem. I can think of two different ways. First of all, as described by @reirab, you can get audio frequencies by interfering two sound waves of high frequency. When they interfere they will generate a beat note which has the frequency of the difference between the two frequencies. I.E. if you send a sound beam with frequency $f_1=200\ \text{kHz}$ and another beam with $f_2=210\ \text{kHz}$, the frequency heard in the region where they combine will be $\Delta f-=f_2-f_1=10\ \text{kHz}$ which is in the audio band of humans.
There is an additional difficulty. You will need the sound to come out in a well-defined, narrow (collimated) beam, and this is not terribly easy to do. A typical speaker emits sound in all directions. There are many techniques for generating such beams, but one is to use a phased array.
How can you use this to make a person perceive the sound as coming from a specific point?
Sending Two Different Volumes to the Two Ears
What does it mean to perceive sound as coming from a specific location? Our ears are just microphones with cones which accept sound mostly from one direction (excepting low frequencies). A large part of the way we determine where the sound came from is just the relative volume in our two ears. So, you could use the interference effect described above with beams which are narrow enough that you can target each ear. By using two separate sets of beams targeting each ear with different volumes, you could make the person perceive the sound as coming from a specific location; at least as well as a 3D movie makes a person perceive images in 3D.
Hitting a Material Which Scattered the Sound Isotropically
The second method is to use the same interference effect, but this time combining the two beams at a point where a material scattered the sound waves in all directions. I'm going to be honest, I'm not sure how realistic such materials are, but lets assume they exist for now. If you did so, the two sound beams would be scattered with equal amplitude in all directions and the person you are trying to fool would percieve the sound as coming from this point. This method has the advantage of truly sounding to the person as if the sound came from that direction in all respects including reflections, phasing, etc.
In summary, the idea is definitely possible (maybe there are more ways than I've given), but it isn't as simple as the passage in the book makes it out to be.
Best Answer
There is a detailed review of this phenomenon here
The phenomenon is known as sonoluminescence. One of the leading theories is that it is caused by "adiabatic heating of the bubble at collapse, leading to partial ionization of the gas inside the bubble and to thermal emission such as bremsstrahlung."