More wracking sobs tear from their throat instead of a spoken response. There was something they were trying to say--another attempt at 'stop' might be caught--but they can't keep their throat clear enough to be intelligible.

Frisk only sort of tries to muffle it. They do want to know what Chara might want, might want from Sans properly. But...they do have a flicker of spite in them when they're furious, and if they want Frisk to be quiet, they're not sure if they really want to be.

And. If they're sobbing and shaking and sounding terrible to their ears...maybe they won't feel Frisk slowly start bracing their feet.

Their eyes keep closed; it's easier to tell what's going on with Chara when they're not being distracted by the sight of Sans anymore.