Is there a karmic reason for my client’s voice ailment or is this due to spirit attachments?

This is karmic and is due to a past trauma. It is not caused by spirits. They will pile on, always, and if nothing else, will taunt the victim and ridicule them, because of their vulnerability and imperfection. They will always bully their host given any opportunity. But on a deep level, there is awareness of the reasons, and the ability to see a wider truth. These things can be healed with the correct approach, and this would be worth taking on.