Quickly adjust/duck audio with the Range tool (Apple)
The Range tool (R) permits you do to all sorts of range based selections and adjustments.
One of the coolest is to use the range tool, select a part of music (or other audio element that you want to duck) and just pull down the Audio level line - FCPX will automatically add 4 keyframes to help you lower only inside that specific range.
FCPX tips like this will be in sessions at NAB/Post Production World