As Alexandra’s entire body will be sealed with her Bane transformation, as well as her mouth and throat being completely filled by the gag, she has no use for her vocal cords. And as the power supply replaces Alexandra’s lungs, her entire windpipe, larynx, and vocal cords, are removed. This makes Alexandra completely mute, unable to create even the slightest sound, due to the missing windpipe and vocal cords.
Although the removal of the vocal cords, windpipe, and larynx change the structure of her neck and surrounding tissue, the insertion of the gag, and later the swelling of her esophagus, actually somewhat restore the previous structure and size of her neck, although it is slightly compressed.