How can I lower background music during audio of video clips

Hello whiskey sour,

This can be done with a feature referred to as 'ducking' (much like you'd duck to avoid a low door frame, the background audio can 'duck' to avoid the video's audio).

To duck the volume of competing audio:

1. In the Project browser, move your pointer
   over the audio clip (not including
   background music) or video clip whose
   audio you want to give priority to, and
   then choose Audio Adjustments from
   the Action pop-up menu (looks like a
   gear) in the lower-left corner. The track you select here won’t have its
   volume changed. Rather, the volume
   will be reduced for any other audio
   tracks that play simultaneously with
   the selected clip.
2. In the inspector that opens, select Ducking.
   By default, the volume of ducked tracks is
   reduced to 15 percent of their original volume.
3. Drag the ducking slider to the right if you
   want the ducked tracks to be louder than
   15 percent of their original volume, or to
   the left if you want their volume to be
   softer than 15 percent of their original
   volume.
4. Click Done.

iMovie '11: Reduce the volume of competing audio clips

http://support.apple.com/kb/PH2269

Cheers,

Allen