The Catcher in the Rye was published in 1951, which explains the excitement Phoebe conveys when she shares with Holden that their family's car now has a radio.
Holden sneaks into his family's apartment in chapter 21 and finds Phoebe sleeping. He wakes her up, and she is thrilled to see him. Immediately, she begins sharing the details of her involvement in an upcoming play; she will play Benedict Arnold, and Holden is happy that his younger sister is so glad to see him.
After conveying her excitement about the play, Phoebe launches into an explanation of the movie she saw that afternoon. Holden keeps interrupting to try to determine when their parents might return home, but Phoebe barely registers his questions. Finally, he presses her to "wait a second" and answer his question. Phoebe replies that their parents won't return until "very late" and adds that "we have a radio in [the car] now!" Their mother has forbidden its use when they are driving in traffic.