See if this applies to you:
Then check out this:
Here is the excerpt:
FAQ #9: Why do does my stream sync sound playback with poor quality?
A1: Flash 5 and Flash MX ignore compression settings in the Sound Properties window for stream sync sounds. Instead, use the Flash tab of the Publish Settings window to set overall compression for all stream sounds, not sound properties window.
A2: Always place stream sounds in the main timeline of a SWF. Upon export, flash compresses all stream sync sounds to the setting in the publish window and merges all stream sync sounds into one long sound. Stream sounds in movie clips do not actually stream and tend to playback at poor quality. These are not valid stream sounds, though they can take on the frame rate control behavior of regular stream sounds.
Note 1: you can load an external SWF into a movie clip of another movie and the sound will stream properly as long as its on the main timeline of the external SWF.