Nobody says 'audi' instead of 'audio'. Regardless of how fast we are speaking, native speakers never drop the last syllable from words ending in 'o'. 'Audio' is always pronounced 'audio'.
What made you think that we pronounce 'audio' as 'audi'?
NB 'Audi' (usually pronounced with the first syllable in as in 'out') is the name of a car brand.