The word “Audi” is Latin, not German. When August Horch lost the battle against his old car company over the Horch name, he settled for the latin translation–Audi.
Despite this, “au” is still a diphthong. There are still two vowels. It should be pronounced as you would say “house” in English. If you listen to yourself (<–hehe) say the word, you will notice that you can make a slight distinction between the two vowels that make up the diphthong. I love this stuff.
And living in a country does not make a person an expert in the language. Look at how many people in America can’t speak proper English, yet are quick to point out that they want it as their official language. Look at how many people that speak Italian and think their language is the closet to Latin, when in fact Spanish retains all of the verb tenses. Aha!
http://www.learn-latin-language-software.com/phrases/player.html?le15.au