He was gagged in the intro. Don't you remember?
His hands where bound, but he could still lift his arms. WHY didn't he remove that gag? That is my biggest question! LOL
Well, he's long abandoned the way of the voice, and he only knows Unrelenting Force. Not to mention he doesn't practice it daily like the Greybeards do.
Surely the cooldown for normal,untrained people must be longer

They could have caught and gagged him while he was on his cooldown!
Ralof tell you why, they were outnumbered 5 to 1, even Ulfric's thu'um couldn't defeat that, he ordered his men to surrender so they wouldn't waste their lifes for notthing, probably he hoped they would only kill him and let his men free because they surrendered
Hmm, ok, good explanations there...
