Audio Problem

I have created a slide show using movie maker that
includes mostly still photos, and a little bit of video.
Following all the directions I found in "Help" I imported
and inserted a mp3 file as music over the still photos.
When I preview the movie from the beginning, I don't hear
my music, but if I stop the movie, and start it again
halfway through, I hear the music then. Any ideas why?
This is my first time using Movie Maker and it's for a
very important project for work and I'm on a tight
timeline. Any help would be GREATLY appreciated!