In FCP 6.0.6 I've mentioned the audio waveform display bug before, but here's a new twist to an old problem.
In the Timeline one set of audio will display the waveform while another one won't. They're identical as far as I can see and in fact, were recorded at the same time only on two separate yet identical cameras.
That's what it looks like in the Timeline. The upper channels are flatlined, but if you load that audio clip into the Viewer you'll see something like this:
Now, if you zoom in one more iteration you'll see the waveform:
Most of the time quitting and relaunching FCP (6.0.6) will clear this up but not always. Trashing the waveform cache can help as well but it's not a 100% guaranteed fix.