Well, what I'd say to that is... he's definitely mouthing SOMEthing, and now that it's been seen, it cannot be unseen. Why would he mouth a three-syllable nothingness in response to Leia?

It makes a lot of sense to move the line to fill in his random mouthed words, which... I dunno. Maybe he misspoke or something, and they cut that part out of the audio to save the take or something, hoping that nobody would notice (which I guess worked pretty well, haha). So even if the audio isn't out-of-sync, there's definitely some kind of awkward mouth movement that looks a lot like he's talking.