To clone a voice accurately using speech synthesis or voice cloning technology, it's important to cover all the phonetic sounds of the English language.
The script used for this purpose should contain all the phonemes of English — known as a phonetically balanced corpus.
One commonly used example is the "Harvard Sentences."
These are a collection of sample phrases that are phonetically balanced and are often used in testing telecommunications equipment and voice synthesis technologies. The Harvard Sentences are organized into lists, each designed to have different phonetic characteristics and to be uniformly distributed in terms of phoneme representation.