Just pick the framerate that you wish to use (should be the same
framerate as your SWiSH movie) ... If you need to make your
own -- it is only necessary to record about 2 seconds of
blank audio.

In your preloader scene -- add this action :

at Frame 2
--- Load Movie (place the name of the blank streaming file
you wish use in the load movie field -- with .SWF on the end
of it -- for the absolute BEST results -- upload this file to your
server -- and place the entire URL into the load movie field --
ie., http://www.yoursite.com/blankstream_15fps.swf)
------- in Level 10 (the actual level doesn't matter - just as long
as it isn't level 0 )

Then, in the first scene of your movie (after the preloader) --
just make your movie like you normally would -- with the audio
playing just like normal (at frame 1 Play Sound yoursound.mp3) ...

Just make sure that you TEST IN BROWSER to see how much
you need to adjust your actions to get the sound to sync up
properly ... It won't work if you use Test in Player -- or if you
test it within SWiSH .. you HAVE to test in browser)