Hannah Pauline Tarley, a ponytailed 17-year-old violinist, smiles for the camera. Then she plays the opening notes of an excerpt from Brahms' Symphony No. 4 as she sways in a room decorated with stickers and posters of the Beatles and the San Francisco Symphony Youth Orchestra.
Tarley filmed herself in her bedroom in Cupertino, Calif., using a computer placed atop several volumes of the Encyclopaedia Britannica. She is one of approximately 3,000 professional and amateur classical musicians in countries from Bermuda to Azerbaijan who auditioned by video in December and January for the YouTube Symphony Orchestra. This...