FFmpeg

.MKVs are popular video files. Unfortunately they will not play on Apple devices. This is silly because the video streams often inside of the MKV container is actually h.264 which is playable on iPhone. Some software will convert these easily, but will take forever and quality is lost.

I use FFmpeg to remux .mkv video files to .mp4 so that they will play with a broader range of devices without further conversion. All of the movies on my NAS as .mp4 or .m4v files.

Windows Command Prompt

Remux all the .mkv files in a folder, stripping the metadata.

for %i in (*.mkv) do ffmpeg -i "%i" -c:v copy -c:a copy -map_metadata -1 "%~ni.mp4"

Same as above, but converting the audio to AAC

for %i in (*.mkv) do ffmpeg -i "%i" -c:v copy -c:a aac -map_metadata -1 "%~ni.mp4"

Same as above, but converting to 2 channels (stereo)

for %i in (*.mkv) do ffmpeg -i "%i" -c:v copy -c:a aac -ac 2 -map_metadata -1 "%~ni.mp4"

Unix shell (Synology)

Remux all the .mkv files in a folder, stripping the metadata.

for i in *.mkv; do ./ffmpeg -i "$i" -c:v copy -c:a copy -map_metadata -1 "${i%.*}".mp4; done

Same as above, but converting the audio to AAC

for i in *.mkv; do ./ffmpeg -i "$i" -c:v copy -c:a aac -map_metadata -1 "${i%.*}".mp4; done