People who suffers from parkinson's disease usually has slow, quieter speech in monotone voice. I agree with kedwards460, it may be related to her medication, it is always best to consult her physician as he is in a better position to provide you with answer and advise as well.
It could be related to her medication or the PD progression. Everyone who has PD is different. My mom who has PD was like your mom about 3 yrs ago. Always talking making "plans" that could never come true. Ask your mom`s neurologist Might need to have meds adjusted.
Might need to have meds adjusted.