Thanks for taking the time to reply. This information is useful.
By reading various blogs I eventually discoveredit was a copyright issue. I followed a suggestion to covert the file to another type and, bingo, the audio was successfully attached after rendering.
The music track I wanted to overlay was from a purchased CD and was being used on a non-commercial family movie. Perhaps this is verboten, but there was no overt indication from iMovie.
Seems to me Apple could easily include a pop-up message to tell you the music file can't be used. It would have saved a lot of time and frustration. Or, better yet, why not a pop-up that allows me to attest the music is for personal use, which then allows the music to attach. After all there is nothing illegal about my playing this same music on my iPod while I show the movie on my TV. How is this different?