The range of frequencies within human hearing, which is approximately 20Hz at the low end to a high of 20,000Hz. However, pundits claim that some people can hear higher frequencies.
Traditional audio devices are analog, because they handle sound waves in an analogous form. For example, radios maintain the audio signal as rippling waves from antenna to speaker. Tape cassette players record sound waves as magnetic waves. In the days of the phonograph record, sound waves were literally "carved" into the plastic platters.
In the computer, audio is processed by converting the analog signal into a digital code using PCM. See A/D converter