Just remembered (advancing senility my excuse) , it was Turandot that had Callas thinking she’d lose her voice.
She actually said that she sang it a great deal at the beginning of her career, just hoping it wouldn't wreck her voice and dropped it as soon as she could. She also called the role Abigaille in
Nabucco a voice wrecker and only sang it once in 1949. She is absolutely stupendous in the role, but maybe she was right. After all its creator Giusppina Strepponi, its creator and later Verdi's wife, retired early practically voiceless having sung the role all over Italy. When Caballé considered singing the role, Callas advised her against it, telling her it would be like putting a precious Baccarat glass in a box and shaking it. "It would shatter," she told her, and Caballé heeded the advice.
There is actually a recording of Callas singing Kundry, though it is sung in Italian, as was the practice in Italy at the time. Vittorio Gui conducts and it is much more than a curiosity. However, if you have an antipathy to Callas's voice
per se then you probably wouldn't want to hear it. I find it the most expressive voice I've ever heard. Her singing may not always be beautiful (though it could be) but it is always beautifully
expressed. Furethermore she was an unparalleled
musician, which isn't always the case with singers.