Do you pronounce every hiragana phonetic in a word every time?
Nope. Your best bet is to learn by listening to how the Japanese themselves do it so you learn how they combine. Try japanese pimsleur or any japanese show.
Ou, hai, hon are all pronounced together
desu is pronounced "des" most of the time. The 'u' is really not audible.
As a counter-example, yaoi is pronounced ya-o-i and not ya-oi (like a pirate).