Zatanna would have to use pretty powerful words from the get-go ('die', etc) whereas it doesn't matter what Bolt says, he could even just do a shout and it would KO her.
She doesn't have to say anything, but the vast majority of her showings show that she does vocally encant to achieve her spells, especially her most effective ones.
We all know Bolt won't go all out and Zatanna has acted quick amongst speedsters before. Hell, more than half of DC has quick reflexes/super speed. If she didn't, she'd be dead by now.
A whisper is still pretty strong - and if they're trying to quickdraw, Z would sacrifice defence for an offence. Unless (you guys prob know the character better) she would go for shields first? Because having basic knowledge means that they know that the other has voice based powers, right? Bolt would know if she says anything backwards, it'll happen....
Looking back at her respect thread, Z is a pretty fast spell caster. Able to turn bullets into whatever AFTER they were shot, toying with Lobo, and easily stopping Despero. Not to mention, she is able to summon powerful entities like Neron and Spectre. I still think Bolt is screwed.
Black Bolt. His words, even a whisper if hes not going all out should do a good amount of damage. And even say that she make it so he can't speak. He is still physically fast and strong enough to beat her.
But just in sheer power of words, Black Bolt.
__________________ Some men, just want to watch the world burn.