Click the line across the audio wave in the clip and drag it up or down to change volume.
Right click on the clip. Click ‘Detach Audio’. You can now move and edit the audio separately from the video.
In the middle grey block on the right side you can click the music button to select audio files. You can drag and drop audio files into the timeline from there.
Other Final Cut Pro X Tutorials:
4. First Steps 10. Synchronisation
5. Cut & Trim 11. Multiple Angles
6. Crop & Transform 12. Anonymisation