The “fade out” recording.
Play it “low” initially and then “high” for the answer
If you combine this fade out with ultrasound (hearing range up to 20Khz then to 100Khz ultrasound) as explained below then you can place it anywhere in space. Anywhere you want: on the rooftops, in front of your face, at the top of a tree. The space around you becomes a speaker but I am not sure where the volume control would be?
Also if you “hear it” or “not” then you “have it” and you “become”.
“The Audio Spotlight & Hyper Sonic Sound Technology (developed by American Technology Corporation), uses ultrasonic energy to create extremely narrow beams of sound that behave like beams of light. Audio spotlighting exploits the property of non-linearity of air. When inaudible ultrasound pulses are fired into the air, it spontaneously converts the inaudible ultrasound into audible sound tones, hence proved that as with water, sound propagation in air is just as non-linear, and can be calculated mathematically. A device known as a parametric array employs the non-linearity of the air to create audible by-products from inaudible ultrasound, resulting in an extremely directive, beamlike wide-band acoustical source. This source can be projected about an area much like a spotlight, and creates an actual spatialized sound distant from the transducer. The ultrasound column acts as an airborne speaker, and as the beam moves through the air, gradual distortion takes place in a predictable way. This gives rise to audible components that can be accurately predicted and precisely controlled.”
(http://www.seminarpaper.com/2011/09/audio-spotlighting-technology.html)
It becomes a “better take it out of your pocket before you forget” shoplifting reminder in some stores these days. Watch you and remind you technology.