In 1823 she sang at Leipzig in Carl Maria von Weber's Der Freischütz and in December of that year created the title role in his Euryanthe.

[1] In 1826, she was engaged at the Paris Comédie-Italienne, where she debuted in the role of Rosina in Gioachino Rossini's opera The Barber of Seville.

In 1852, she toured America, and in May 1854, at a literary evening in honor of Mexican president Antonio López de Santa Anna, she made public for the first time the lyrics that Francisco González Bocanegra had written to celebrate the nation (with an Italian musical arrangement).

Hector Berlioz wrote of Sontag: "She unites all the qualities—although not in an equal degree—all like to find in an artist: sweetness never surpassed, agility almost fabulous, expression, and the most perfect intonation.

How fortunate for our young singers that, like the nuns in Meyerbeer's Robert le Diable she left the tomb of the seven ancestors, bestowed by the King of Prussia upon the Countess de Rossi, to teach them the wide difference between singing and screaming, and to show how we all, during the last ten years, have been listening to, and adoring false prophets.