Problems with WIndows Movie Maker?

There is a good chance that the timing of the song versus the timing of the videos/animated clips are not in sync. The song will end when the video does, so you need to check the lengths of the video and add a few screens to make it end the same time that the song does. Keep in mind that most songs running full length are between 3.5 and 4.5 minutes. You can also add a credits section to the end to help bring the video to the end of the song, too which I have found to be the best method. Hope this helps