If you're still thinking about causes for her disability, and she was meant to be advertising her product, maybe she was given some sort of mod to help her shout advertising louder, or longer. Or maybe to have a more attractive voice so people would be more likely to listen to her. That would be a reason for her to get a speech-related modification to "go wrong" and render her mute. And having it be from something that was supposed to help her speak better would add that element of irony.
Though I don't know if this fits when her disability occurred, how you want her disability to have happened, how mods are used, or how mods malfunction, or why the problem couldn't be fixed.
Though I don't know if this fits when her disability occurred, how you want her disability to have happened, how mods are used, or how mods malfunction, or why the problem couldn't be fixed.