Hey sponge. My Dad has had PD for almost 20 yrs now, and I've noticed that there are times when his speech is definitely impaired. Sometimes it just sounds as though he is mumbling, and I can't make out the words. It tends to happen more and worse when he is tired or stressed. Also if his meds are "off" it can get bad. I really don't know what to have him do about it. But like I said, it doesn't seem to be as bad if he is rested and relaxed. Hope this helps some.
Perhaps a speech pathologist might help or be able to give advice. Our brother is in end stage parkinson's. Several months ago they evaulated his speech and swallowing ability. They did a vidieo which was very informative.