All clips have an audio track, and on clip one and two, the 'visual' for the audio track has the audio waveform shown clearly, then in clip three and all ther rest the waveform is 'flatline' (dead), but the audio is there and audible from the speakers. An earlier edit with a similar amount of clips (about 8) was fine. I could not figure out why the waveform went away, nor how to restore it...... All hints welcomed..... Thanks
Sounds like the audio didn't get a chance to conform.
Right-click on it in the Project panel and select Clear. Then reimport it back into your project. Wait a minute or two for it to conform before you use it.
I agree with Steve. Editing, especially using a Clip, before Conformation is complete, can often cause what you describe. This ARTICLE goes into more details on Conforming.
Good luck, and if that is the problem, just Delete that Clip, and Import again, allowing Conforming to complete 100%.