noun / adjective
audio
AW-dee-oh/ˈɑdiˌoʊ/
Sound recorded, transmitted, or reproduced.
The equipment records audio and plays it through headphones.

Etymology
The root meaning “to hear” supplied a word-forming element for sound and hearing. Audio became an independent noun for sound, especially signals recorded or transmitted electronically.
In context
audio is half the viewing experience especially in a documentary where a lot of times you're just putting b-roll over top of the images that audio is actually what's carrying the story