Mechanical vibrations of what frequencies are called sound and why?

First, those vibrations that occur in any medium (gas, liquid, solid) that transmits sound. Secondly, having a frequency range from 16 Hz to 20 kHz, audible to the human ear.

Below 16 Hz, vibrations cease to be sound; these are long-wave infrasonic vibrations. A person does not hear them, some animals, for example whales, hear them.

Above 20 kHz, there is a spectrum of ultrasonic vibrations, which is also inaudible by humans, but audible by some animals (for example, dolphins and bats).