Centering audio in Final Cut Pro X
First, make sure you have a stereo project (timeline), unless you are dealing with 5.1 (surround) files:

Select the clip, and at the property inspector set it to dual mono:

Now both tracks will play at both channels.
By doing that you will also enable breaking apart clip itens (command+shift+G):

Tracks are now independent from the video.

You can manually map each one for whatever speaker you want.

Just remember you can't break apart all the channels before setting them to mono. If they are grouped you will just detach it from the video.
Also, expanding the audio tracks (option+control+s) won't do pretty much anything regarding channels, it is just a tool for visual aid and some editing purposes.