I struggled with this issue for the past few days before I figured out two possible solutions that haven't been mentioned yet in this forum.
One, relink your files. This one seems obvious but this is the fix that had me stumped for days. I had already relinked all the files when I opened the project. Or so I thought. But then a few hours later, just for giggles, I tried another relink. That time, it found even more files that needed to be linked. That made no sense to me because ALL project files were glommed together in one folder -- no subfolders. So, why didn't it pick up all the files the first go around? Whatever. After the second relink, more files appears in the timeline, but I still was lacking the audio for about 10 clips.
After fiddling around for another hour, I found a second option for fixing this issue. Right click your audio clip and hover over the "Active Audio Angle" option. If there's more than one source or version of the audio file associated with the video clip (for instance, if you used a shotgun mic OR if you PluralEyesed the audio to sync it up), you can select those other audio files to use with your video.
That of course was not the final fix for me, though, because the other audio files were not as good as the PluralEyes version I wanted to use (which, for some reason, was present with the video in the timeline but was lacking any presence of audio levels). So, what I ended up doing to FINALLY get this audio issue resolved was I relinked files for the project a THIRD time and FINALLY it got the last few bits linked up.
How crazy is that? I had to relink files THREE times in ONE folder to get all the files.
The Verdict Is: Forget Final Cut Pro X. Switch to Adobe Premiere. It is WAY more intuitive and user friendly.