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.
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.
Might need to have meds adjusted.