Rather than dropping music tracks onto the background area, I find it more efficient to drag each track directly onto a video clip. Instead of filling the whole background, a thin green bar (ribbon) will appear under the video thumbnails after you drop the track. The bar can be dragged anywhere in the project. The ends can also be dragged inwards to shorten the track (and dragged out again if necessary). You can also view the waveforms (if turned on) and easily adjust volume levels and fades within the timeline.
This method means that you don't have to use the "Clip>Arrange Music Tracks" menu item (in fact it is greyed out if you don't have any floating background music). In iMovie, the bars/ribbons are referred to as "voiceovers" and can consist of music or, as the term implies, voiceovers (recorded speech using a mike). The bars can be either green or purple, depending on their source.
John