She was given speech therapy, which involved the Hayeses (her adopters) manipulating her lower jaw.
Eventually, she was able to voice four words: Analysis on recordings of her vocalizations concluded she was "incapable of simultaneous recruitment of consonantal “frames” and vowel-like 'content' but she "successfully produced a small sample of humanlike consonantal speech sounds, including labial and, possibly, velar articulations".
[1] At the time, her extremely limited success was initially interpreted as showing that apes were incapable of using human language.
However, further experiments in which chimpanzees were instructed in the use of American sign language indicated that Viki's achievements had been significantly hampered by physiological limitations—chimpanzees are not able to produce the sounds that make up human speech.
The immersive language experienced failed, however, after three years Cathy Hays said “the only obvious and important deficit in the ape's innate intelligence, as compared with man's, is a missing facility for using and understanding language” (Fitch, 2002).