Audio randomly disappears

Audio on various clips disappears on playback. Selecting
the clip, Ctrl U to open audio dialogue and clicking Okay
will get the audio to work at that point in time, but
when re-viewing the movie the next time; again there will
be no audio on that clip.

The movie exported back to the camcorder includes the
sound on all clips.

Saving the movie in any other format results in no audio
again on random clips.

Why is this happening and how can I correct it.

Terry B