Short answers, pulled from the story.
In physics, sound is a phenomenon in which pressure disturbances propagate through an elastic material medium, characterised as a mechanical wave of pressure or related quantities such as displacement. In physiological and psychological contexts, the same word refers to the reception of those waves and their perception by the brain.
The human ear is sensitive to frequencies ranging from about 20 Hz to 20 kHz, and the upper limit decreases with age. Below about 20 Hz, periodic acoustic stimuli are perceived as discrete pulses or slow amplitude fluctuations rather than as pitch.
In 20 C air at sea level the speed of sound is approximately 343 m/s. In fresh water it is approximately 1482 m/s, in steel about 5960 m/s, and in solid atomic hydrogen about 36000 m/s, the fastest known medium.
Sound cannot propagate through a vacuum because there is no medium to support mechanical disturbances. Sound requires a transmission medium, which may be any form of matter, whether solid, liquid, gas, or plasma.
Ultrasound is sound waves with frequencies higher than 20,000 Hz, used in medical diagnostics and treatment with devices operating from 20 kHz up to several gigahertz. Infrasound is sound waves with frequencies lower than 20 Hz, detectable by whales and elephants and usable to detect volcanic eruptions.
Isaac Newton made the first significant effort to measure the speed of sound, believing it equalled the square root of pressure divided by density. The French mathematician Laplace corrected this by recognising sound travel is adiabatic rather than isothermal, adding the factor gamma to produce the Newton-Laplace equation.
Historically there are six experimentally separable ways sound waves are analysed: pitch, duration, loudness, timbre, sonic texture, and spatial location. Pitch reflects the frequency of vibration, timbre gives a sound its identity, and loudness reflects the overall pattern of auditory-nerve activity.