What does the quote, "They're such beautiful shirts, she sobbed, her muffled in the folds. It makes me sad because I've never seen such beautiful...

In this scene, her voice is "muffled in the thick folds". This describes her mixed feelings of being impressed and full of regrets at the same time.

The shirts are the juxtaposition of her realization between what currently is - her materialistic unhappy self - and what could have been - her romantic blessed self.

Currently, she is married to Tom, her cheating husband, whom she does not really love. They are only married because of his inherited wealth and the promised security and stability it would give her.

What could have been is a truly romantic relationship with once-poor-but-now-wealthy Gatsby. The shirts are being described as colorful and unique, which is what she thinks their life might have been.

Daisy chose money over true love and realizes in this moment that if she had listened to her heart, she would have ended up having both. That's why she finds them so beautiful but is at the same time sad.