Adding music to a Movie Maker Video

My daughter and I are making a movie.  She has two segments, each with multiple clips.  The main movie is the first segment, and the second segment is called the gag reel.  She has inserted some music into the second segment which is perfectly synced with the clips.  When she tries to add music to the first segment, it changes where the music starts in the second segment.  Are we doing something wrong?  The music she want to add to the first segment is shorter than the segment length, so it shouldn't "push up" the start of the second song, but it is.  Also, whenever she makes any changes to the first segment, that changes where the music in the second segment starts.  What are your suggestions?

Thanks!