Audio refers broadly to the information, be it acoustic or electronic, entering, traversing, or exiting some medium.
Audio has the meaning of being "related to sound." This broad topic can be subdivided into audio signals, which are electronic representations of sound, and acoustics, which deals with the mechanical transfer of sound waves through air.
In the context of this site, audio generally refers to the recording, manipulation, or reproduction of acoustical signals. This includes both professional and hobbiest recording, different tools and techniques related to the manipulation of audio signals, or equipment and layout for accurate reproduction of sound.
Examples of questions that would warrant the
Is there a noticeable difference between [mic x] and [mic y]?
How loud should my mix be before mastering?
How should I place my studio monitors to get the most accurate stereo mix spectrum?