Unlike in English, there are
only 5 vowel sounds in Japanese:
ah, ee, oo, eh, oh
There are
no exceptions to this rule.
There are, however, exceptions to how to pronounce the characters.
ア, エ, and オ are always pronounced the same.
When イ follows the エ sound (せんせい) the イ is pronounced as an エ.
When ウ follows the オ sound (おとうさん) the ウ is pronounced as an オ.