All three of them went to Moon Lake Casino, and danced the polka. As Blanche retells her memory, she hears the music again. In the middle of their dance, the boy broke away and shot himself. When Blanche describes hearing the gunshot, the music stops. You disgust me.
Blanche begins to hear the polka music again.
As Mitch embraces her and tries to comfort her, the polka music fades away. We also see the act that Blanche puts on for Mitch. She pretends to be taken with old-fashioned values. Her need to be the virginal Southern belle isn't malicious in intent. She indulges in the deception not only to attract Mitch, but for her own sake.