I finally chose to use Handbrake to do the mkv conversion. I found the audio output option to convert audio to ac3 (ffmpeg) and selected this. The file now plays.
I did not try this on earlier comments that Handbrake would apply additional compression. It seems though to have worked perfectly. A little inconvenient to convert movies at 1/2 to 1/3 play time, but the PC can work while I sleep.
thanks for the help.
