The qusestion was brought up in a number of places on Divrei Chaim, including:
Now we'll take a different perspective that my grandfather presents. His interepretations of Vatisachek Sarah bekirba [Sarah laughed within herself] signifed that Sarah was herself unaware of the laughter that arose within. It was not just inaudible but unconscious.
It was only in the depths of her soul that there arose some glimmer of doubt about the truth of what the angels said because acharey baloti hayta li adena [after I've grown old/worn I've become rejuvenated]. In this interpretation, leymor [saying] does not signify a quote but the reason. Sarah would not have said say those word…