The quick way:
Click on the clip (or press ‘c’ to select), in the audio section of the inspector select “dual mono” instead of “stereo.” If you just don’t want a track at all, you can deselect it. If you want to independently edit the audio tracks, right click the clip and select “open in timeline.”
You can now edit the tracks ala all other NLEs on the planet. (press command-[ to go back to the main timeline)
A nice quick edit feature:
Select range mode (press ‘r’), then highlight a section of audio that you want to raise or lower volume. In the middle of the clip you can drag the levels up or down and FCPX creates the appropriate keyframes. Or press delete to cut a gap.
You can manually add keyframes by option-clicking…