Strangely, I was not able to re-assign roles to that audio. I just wouldn't let me. BUT, even though I didn't find out exactly what the problem was I did manage to make it work. Here's what I did:
From the Finder, I dragged that same audio file (which is in WAV format) into my library. Immediately I noticed that the yellow triangle disappeard! BUT, the audio clips were still grey bars in the timeline.
Out of a hunch, I quit FCPX, relaunched and Voila! The grey bars became the beautiful blue again!
I suspect that there is something funky with the WAV audio file. I recommend to anyone having this issue to convert the WAV to AIFF and work with that instead.
I wanted to report back in case this helps anyone in the future (if it does please leave kudos).