[4] In 1931, she sang Norina in Donizetti's Don Pasquale at Bielefeld Opera, where her voice made a great impression and her gifts were immediately recognised.

In 1934 she was engaged by the Breslau Opera, where her roles included her first Zerbinetta in Ariadne auf Naxos, and the following year arrived at the Semperoper in Dresden, where she attracted the attention of Karl Böhm and, above all, Strauss.

In 1934 she also made a spectacular return to Berlin, appearing as Gilda in Verdi's Rigoletto alongside Heinrich Schlusnus as the Jester and Walther Ludwig as the Duke under the baton of Erich Kleiber.

She seemed to work tirelessly, at the opera, in concert tours, and touring, including to Rome where she appeared in Mozart's The Magic Flute with a cast that included Tito Schipa and Licia Albanese, Copenhagen, Oslo, and, for the first time, the United States, where she shared a platform at Carnegie Hall with Joseph Schmidt and Richard Tauber (24 October 1937) singing a duet from Lehár's The Merry Widow.

She had some difficulty, however, when asked to sing at the Lyric Opera of Chicago, being asked to sing the roles of Rosina and Donizetti's Lucia in the Italian language, because she argued that she had not had sufficient time to re-learn those roles in their original language (throughout Europe at that time operas were largely sung in the tongue of the nation in which they were being performed).

[1][2] After World War II, Sack toured extensively and was particularly successful in Latin America, especially Argentina, Uruguay, Chile, and Brazil (with the result that she and her husband took Brazilian citizenship).