You must have beaten the Old Lady from the Dining Room to get the Cassette Tape. You also need the Study Key from the Sink in the Kitchen. You can get the Study Key by using the Coat Hanger on the Sink. You get the Coat Hanger from the basement closet.
You use the Cassette Tape on the Computer in the Study on the search bar under "Music". This allows you get the song the child wants to hear downloaded on your cell phone. Play it for him in the Foyer.