The electrical representation of sound in analog or digital form. Whereas "sound" is heard, "audio" cannot be perceived by human ears as anything meaningful. Audio does, however, refer to the range of human hearing frequencies, which is approximately 20 Hz to 20,000 Hz. Some people claim they can hear higher frequencies.
Audio Is Not Audible
Technically, audio is not "audible." Only sound is audible, which is illogical terminology but why the terms "audio" and "sound" are used synonymously all the time in casual conversation.
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 a vinyl phonograph record, sound waves are literally "carved" into the plastic platter.
In the computer, audio is processed by converting the analog signal into a digital code. See A/D converter