nia use ffmpeg to output back to the primary audio device
Yes, that's much more sensible than what I was contemplating: using
hdaudioctl to split out speaker+HP into separate speaker & HP devices (and, not sure here if will get 2 devices); or do the split earlier at the source.
This should've worked, but, the "monitor" playback is very choppy:
ffmpeg -thread_queue_size 2048 \
-f s16le -ar 44100 -ac 2 -i /dev/pad \
-f oss -i /dev/audio \
-filter_complex '  amix' \
-f x11grab -r 30 -s 1366x768 -i :0 -vcodec libx264 -preset ultrafast \
output.mp4 -map 0:a -c:a copy -f oss /dev/audio
And, the example in the guide doesn't work at all.