Then , in 1928 , he got a part in the jazz opera Jonny spielt auf by Ernst Krenek when it was performed in Paris . The next year he sang for the first time in the United States in NoÃ `` l Coward 's Bitter Swee t. He worked in Geneva until 1933 , then he went to Paris where he met the music teacher Nadia Boulanger . In 1928 , he made his stage debut in Ernst Krenek 's Jonny spielt auf in Paris , and in 1929 he sang for the first time in the United States in NoÃ `` l Coward 's Bitter Sweet .