In terms of Parkinson's and speech, I have read and also watch my husband's speech become much softer. After hearing a specialist talk on how Parkinson's can affect the vocal cords, I had my husband sign up for speech therapy. This disease can also affect the ability to swallow as it progresses.
In speech therapy he practices speaking louder, and doing various vocal strengthing exercises. In our area we are fortunate to have a twice a week exercise group for those diagnosed with Parkinson's.