"Cô ấy trở nên yên lặng."
Translation:She becomes quiet.
This is what I think it is a few months later (can confirm later):
yên lặng: adj used for describing someone or something as being quiet
yên tĩnh: adj used for describing a place or event as being quiet
Ie. Rowdy kids and telling them to be quiet - yên lặng. Ie. Empty church/temple - yên tĩnh