Windows movie maker video problems please help??!?

ok so im making a video on movie maker, a video with a song playing in the background, but it seems that i am getting problems with the length of the song, the song that i have put on it is 5 minutes long but movie maker says it is 8 minutes long, this has happened to me before, i made a video and the i put enough clips to fit a whole song, then when i play the video in wmp, the song finishes before the video does and i have to add an extra song to make it up, please can somebody tell me how to sort out my problem...