I’m going to guess that you have a mySound.start() sitting on a movieClip or a different layer or else it won’t start. The reason that when you set streaming to ‘false’, Flash will load the entire sound before playing it. There should be no difference in quality when externally loading the mp3 unless there is a problem with your mp3. Check this out:
I may have the code in the wrong place. For example, imagine, you opened a new Flash Document and the code is placed in the 2nd frame. That’s where I put it.