I re-ripped both movies to MKV again -- making absolutely certain I'd checked the audio stream boxes -- but still the same thing...no audio when streaming to my Oppo, and the same VLC error message when I tried playing them (which I usually do to test/preview an MKV ).

Is this a known problem with some blu-ray movies? I've ripped over 25 of my BD movies to MKV with no such issue...and ripped a different blu-ray movie to make sure nothing had changed -- and the resulting MKV had the all the audio tracks.
Any clue would be helpful.