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.
あ, え, and お are always pronounced the same.
When い follows the え sound (せんせい) the い is pronounced as an え.
When う follows the お sound (おとうさん) the う is pronounced as an お.