Complex sentence: Because she had such limited control over her voice the length of time she held out a vowel was initially the only stress indicator Genie used, and until she incorporated other stress indicators into her speech, the length of a stressed vowel was very exaggerated.
Split into simpler sentences: Because she had such limited control over her voice, the length of time she held out a vowel was initially the only stress indicator Genie used. Until she incorporated other stress indicators into her speech, the length of a stressed vowel was very exaggerated.