I am editing a video slideshow on Windows Movie Maker 2012. I have added several .MP3 files to the video as background music, however when I preview the video there is no sound. They are correctly placed in the video, with the sound levels on 100%. My speakers are working. I get sound from every other program running on my computer. Windows Movie Maker 2012 even makes some sounds with the animations & things I've added, however, there is no music. None of it works & I want to know why. How can I make it work? 
Thanks!