This is my idealogical article..
This is a phenomenon by which an ultrasonic mechanical wave loses its energy stored in the form of frequency shift when it travels long distances.
Thus an ultrasonic wave after traveling long distance looses a considerable amount of energy through frequency attenuation would fall in the audible range.
This concept is similar to how the amplitude of sound waves gets attenuated when it travels long distances.
A musical in the audible range encrypted in the ultrasonic band, could be transmitted just like a sound wave to a long distance. There it naturally reduces to audible range.
Thus without the need for a receiving station or a speaker, sound can be transported.
It would appear as is sound coming from nowhere.