Related a bit to the missing audio waveform bug is something I experienced today in FCP. I was on the phone talking to a client when right before my eyes an external FW hard drive decided to unmount itself.
Naturally FCP flipped out and claimed -rightfully so- that everything was unconnected media and colored everything red in the timeline. While I still don't really know what caused the hard drive to unmount itself while nothing was going on, it did remount itself an instant later.
FCP saw that everything was reconnected but for some reason the clips immediately displayed in the timeline still had red missing media thumbnails. Quitting and relaunching FCP and the project didn't help (I didn't try restarting the Mac) so I quit FCP and deleted the two thumbnail caches at ~/Documents/Final Cut Pro Documents/Thumbnail Cache Files/
To find out where yours are (if not default) looks at FCP's System Settings.