How to remove backgrounds in iMovie?

Every time I get my movie the way I want it and add music from my I tunes library and then share it to a file. When I cli on the file I get the same stock music and not the music I have selected. What am I doing wrong, as I have never had this problem before.

Stack the clip you want to be the pip on top of the timeline clip. Select the clip and click on the adjust button above the viewer. Click on video overlay settings and select picture in picture from the left most popup.