If we assume that someone is giving the pass-phrase they've selected in their voice, then it does make the guess more expensive. Now there are two things to gue

If we assume that someone is giving the pass-phrase they've selected in their voice, then it does make the guess more expensive. Now there are two things to guess: What they sound like saying a particular thing and what that thing is. Of course if you have a secure pass-phrase you're not gaining too much there. It's already very difficult to break. But ho-hum, it's not automatically a terrible idea. Someone could record you saying it, but on the one hand I don't think that you're as much at risk from someone around you recording your password as you are from someone trying to guess it, and on the other this is a vulnerability that other forms of password share too - though I grant with a slightly different recording procedure.