Most of my voice problems are caused because I cannot get the air out to "push the words" in normal speech. Not because the muscles used to speak are going or have gone bad. So I have a very weak voice that as the day goes on gets weaker and squeakeir. The doc told me that if you can do a quick shout, then it is breathing muscles if you can not then it is vocal muscles.
That said, when I use my BiPap/Trilogy I do notice that my voice gets stronger for a little while afterwards, because my breathing gets better. The same is true when I get up in the morning.