Upon singing "Happy Birthday" in all 12 keys, Rossum was welcomed to join the Metropolitan Opera Children's Chorus by chorus director Elena Doria at seven. Over the course of five years, she sang onstage with the chorus and had the chance to perform with singers such as Plácido Domingo and Luciano Pavarotti. For anywhere from $5 to 10 a night, Rossum sang in six languages in 20 operas, including "La bohème", "Turandot", a Carnegie Hall presentation of "La damnation de Faust", and "A Midsummer Night's Dream". She also worked under the direction of Franco Zeffirelli in "Carmen". An increasing interest in pursuing acting led to taking classes with Flo Salant Greenberg of The New Actors Workshop in New York City. She also has worked with acting coach Terry Knickerbocker.