We don't pronounce 'u' in 'arigatou', pronounce like 'arigato-' as stretching the end of 'to'.　ありがとぉー。 That'd sound more natural.

To be more accurate the correct way of reading the word in English is like this "Arigatō". The line above the "o" means that it has to pronounced longer than usual. There are many words in Japanese that uses this kinds of words. Another example is "どういたしまして". Although it looks like "Do u i ta shi ma shi te", it is heard to be pronounced without the "U"; actually the "O" on "Do" has to be pronounced longer. (Dōitashimashite) Hope my answer was helpful too. Have fun learning Japanese! m(^ ^)m

"-ou" is always pronounced like a long o, "-ei" always like a long e. See e.g. "The Long Vowel Sound" here: http://www.guidetojapanese.org/learn/complete/more_sounds Additionally, う (for example inく) is weak and often shortened to something like a schwa (http://en.wikipedia.org/wiki/Schwa) or not spoken at all. Same for い, especially in し.