Lets say your footage was shot with all kinds of variances in the levels. Things are mixed incorrectly between channels, or your cameraman decided to bump up one channel to "save" it in case the audio peaks so you're left with uneven audio that changes levels between takes. What's to do?
In your timeline...
01) Highlight all of the audio that needs to be adjusted.
(Optional - Tap Option + L to turn the audio into Stereo Pairs if they're not already since this makes adjusting levels easier later on if you have to keyframe it. You'll know the audio is a stereo pair because you'll see sets of triangles in the audio clips that weren't there before)
02) Tap Control + . (period) to bring all of the audio clips to Pan 0 (center)
03) In the menu bar (yikes no key command? Not by default, sadly) head over to Modify > Audio > Apply Normalization Gain
04) Choose a value appropriate for your audio. If it's dialogue try setting it to -12 or -14 and see how it sounds. If it's background music try setting it to something lower like -20 or -25.
Let FCP render this out and see how it sounds. Adjust as needed.
05) Back up in the menubar hit Mark > Audio Peaks > Mark and let FCP analyze your timeline for audio peaks. It'll place markers in the timeline (not on clips, wish there was an option for this though) where the audio levels hit 0 or louder. Make sure you adjust these with keyframes to bring them back down.