The prosecutor is only partially correct in that humans have two sets of vocal cords (they are properly known as vocal "folds"). He calls them "duel sets," consisting of the "superior vocal cords" and the "primary ones." They are correctly known colloquially as "true vocal folds" and "false vocal folds." The FVF are called "false" because they are made up of membrane, whereas the true folds have a deep layer of muscle tissue and can be controlled. The FVF can be recruited by powerful airflow and/or by disciplined muscular movements by the muscles surrounding them. However, they cannot be "activated" in the sense that a muscle can, and would not produce a different "voice." At most, some harmonic overtones or vibratory interference (such as that heard in Tibetan chanting) might be heard. The prosecutor uses the term "dual voices" as if it means two separate actual voices, as if "voice" was being produced by two distinct sets of vocal folds, which is not possible in humans. The writers confused it with some individuals' ability to produce two different fundamental frequencies by vibrating each of the true vocal folds at different rates, but the act of forming words is not determined at the vocal fold level, but by resonances created by the positions of the articulators in the vocal tract.
When Erin Bruner is awakened at 3 am by the exorcism tape playing, she goes to push stop. But when the tape player is actually seen, the reels are not turning, therefore when she presses the stop button, the reels remain unchanged.