Thank you so much!
When I tried to make AvsGunnar's command below (that worked on a single file)
---ffmpeg -i roku.mkv -vcodec copy -acodec aac -strict -2 newroku.mkv
into a 'batch' command to process all files in a directory, (and I can't remember the "incorrect" command I used, I got syntax errors I didn't understand, so took out the -c:v copy (or -vcodec copy)).
adding the -c:v copy syntax took the conversion speed up to 28x
for %f in (*.mp4) do ffmpeg -i "%f" -c:v copy -c:a aac "%~nf.mkv"
Again, thank you all from this 'newbie'
ALSO
it took me a while to find how to identify the CODEC info for the Audio- I finally found it using VLC Media Player. If other newbies aren't aware of how to find this, an easy location is using -- VLC Media Player-Tools->Codec Information
