audio, pronounced /ˈɔːdioʊ/. Spelled a-u-d-i-o. Please check the audio before the meeting starts.
The spelling 'au' represents one vowel sound, /ɔː/, as in launch, so the 'u' belongs; dropping it (adio) is wrong. The final 'o' is /oʊ/, as in no.
Using it
A small trap: some learners write adio, dropping the silent-feeling 'u'. It is not truly silent in sound terms, since 'au' spells one vowel, /ɔ/, so both letters belong. Spelling it audo or audeo is simply incorrect.
In British English the first vowel is often /ɔː/, and American speakers may merge it with /ɑ/, so audio can sound like /ˈɑːdioʊ/. The spelling stays the same across varieties.
The word 'audio' is not tied to any one register. It fits equally well in casual speech and in formal settings like meetings or presentations, so you can use it naturally in either one: The audio is too quiet.