Even though FCPX is fairly decent in terms of its capability in editing audio, in many instances you'll want to use a more powerful and nuanced program for this. In the Mac environment, that program would be Logic Pro X.
This tutorial will walk you through the steps for importing your FCPX project into Logic (where you can utilize its sound library and mixing and mastering functionality to elevate the quality of the audio in your project), and then send your audio mix back into FCPX, where you can marry your mastered audio to your video.
(Note: this tutorial will not show you how to actually mix and master audio! That is a HUGE topic and will be addressed in a subsequent tutorial, or in a suite of tutorials. This tutorial is specifically focused on the workflow for round-tripping your project from FCPX, to Logic, and then back to FCPX.)
Here is a video that shows step by step how to do this: