It depends. If they are talking and it is barely audible, sure...I do this, and with the exceptional expletive, it's more or less silent. Most of the developers here, myself included, wear headphones for the day unless we're talking to one another. I don't think anyone even notices I'm mouthing things to myself or reading "out-loud" (e.g., moving my mouth).

Not totally related, but I'm also bobbing my head to music all day (usually groovesalad on some.fm)

All that said...how would you know if you're going to hire someone who talks to themselves anyway, or is this just hypothetical?