Where thoughts serenely sweet express (line 11)
The sound of this line, with its repeated s sound (a.k.a. "sibilance"), reflects the meaning of the line as well: the line sounds as soothing and "serene" as her thoughts supposedly are.
How pure, how dear their dwelling place. (line 12)
The speaker assumes that the calm and "serene" expression on the woman's face reflects the "pur[ity]" of her mind.
But tell of days in goodness spent, (line 16)
The speaker also assumes that the woman's sweet expression "tell[s]" the story of her past goodness – that's a lot to read just from someone's facial expression.