Answer:
The lines in this passage that are a flashback are :
She thought of her last day in middle school, seven years ago, when Sarah had given her an iPod. She had felt ecstatic that day when she realized she would be able to listen to her favorite songs and fend off comments from her cousins about how little she knew about music.
Explanation:
A flashback is a disruption in the order of the events in the story where the character remembers a past event in order to explain a present situation or to give background context to it, in this case, the lines describe a moment that is connected to the music as she is listening to it in the present moment at the party.