Use the fill left (or right) audio effect. This will duplicate the good channel on to the empty one.
Thanks, Richard, for your reply. I'm not at my office now, but can I assume the following:
(1) Select the clip in the timeline
(2) In the Effects panel (not the Effects Controls), locate the audio effects
(3) Do the left or right fill
Am I on the right track (no pun intended)?
Yes, or you could add it as an effect in the track mixer.
Thanks for the help. It worked perfectly.