ffmpeg: From (soundless) mp4 to mp4

Used this to transcode a video file (an mp4, but nevertheless)  that played on the Roku3 without sound:

./ffmpeg -i My.Movie.mp4 -c:v copy -strict experimental -c:a aac -ac 2 -ab 256k My.Movie.2.mp4

It took less than 15 minutes to process the file on the T430.