As animals vocalize, their vocal organ transforms motor commands into vocalizations for social communication. ~ Danish zoologist Coen Elemans et al
Sound is a vibration propagating as an audible wave of pressure. Audition is sound perception.
Animals make sounds in a variety of ways. The subtlest and most complex is via vocalization: willfully pushing air through the respiratory system in a precisely controlled manner.
Humans and other mammals vocalize through their vocal cords, which are more accurately called vocal folds: 2 folds covered in a mucous membrane which extends across the interior cavity of the larynx, which is the hollow tube atop the windpipe (trachea).
The vocal folds are twin infoldings of 3 distinct tissues. A central, thin, loose, gel-like layer of connective tissue, the lamina propria, forces a vocal fold to vibrate and produce sound when muscles regulate the folds as air from the lungs rushes past.
The lamina propria includes an array of different cells, some of which form intricately arranged and layered elastic fibers. A pair of vagus nerve cables physiologically provide the necessary stimulation to twitch specific lamina propria fiber cells so that they stretch and tighten with molecular precision. Dissection has failed to reveal how this exactly works, especially in light of the ability to punctiliously adjust pitch, intensity, and timbre at the millisecond interval.
Birds vocalize through their syrinx, which has no vocalizing membrane. Instead, avian vocalizations are made by muscling the walls of the syrinx during airflow. They can do so at over 10 times the speed at which humans can vocalize. The physical structures of avian syrinxes differ considerably.
The syrinx is a tiny box of cartilage. It reinforces the airway, and when air passes over the folds in it, it produces a sound: birdsong. ~ American zoologist Chad Eliason
Despite using disparate organs independently evolved, mammals and birds converged on the same mechanism for vocalization, corresponding with the myoelastic-aerodynamic theory.
Expiratory airflow is mechanically converted by vocal folds into pulse-like airflow, which causes air pressure disturbances constituting the acoustic excitation of the system. The mechanical properties and recruitment of different layers of vibrating tissues affect their resonance properties, which in combination with aerodynamic driving forces determine the frequency and mode of oscillation. ~ Czech biophysicist C.T. Herbst et al
While sound is merely a mechanical pressure wave at a frequency which is audible, the specific process of vocalization is astounding when considering the precise control which animals must have to produce the exact sounds that they do. Sound-producing tissues must be continuously moved with molecular precision while meticulously adjusting airflow.
Just like vision, the everyday activity of utterance is so mundane that we take it for granted as being physically accomplished. Considering the precise atomic intricacies involved, it is impossible to imagine that twitched cells and physical forces alone make vocalization happen. Modulated energy must be involved, and energy is not material, even as it has physiological effect.