Dear Horsegoer,

I should think muscle degeneration is causing this voice change. Our voice is a strong muscle that can be exercised to the point of being an opera singer. Unused the muscle starts to weaken. (This is the reason for losing your voice when you have a throat infection or if you are really nervous, your voice can dramatically falter). So when the voice muscle is weakened through a disease, the voice will naturally change.
Balance issues are a well known part of the disease. it's called "retropulsion" and is due to the natural ability to steady oneself deteriorating. But unsteadiness worsening could also simply be anxiety at this new sensation and the fear of falling.

I wish you well.