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Avidemux is available for Linux, BSD, Mac OS X and Microsoft Windows under the GNU GPL license. The program was written from scratch by Mean, but code from other people and projects has been used as well.
We have already discussed How to install avidemux in ubuntu.
Procedure to follow
1) Open the mkv file in avidemux and Select safe mode if prompted.
2) Leave the video as copy.
3) Choose File-> Properties. Check if the audio codec is aac.
4) If audio codec is NOT aac, select aac(faac).
Otherwise, leave it as copy. (Sometimes using this option the audio may be out of sync or distorted, in that case select aac(faac) instead.)
Select mp4 for format.
5) Select save, choose location and type a filename with .mp4 as the extension.
To get a MKV container in the first place, you can use HandBrake (open source) for dvd convert and compression (ffmpeg or x264). It will also go to a mp4/m4v mobile format, as part of their presets.
For those difficult to read dvd/blu-ray disks makeMKV linux beta (monthly beta key free, while in beta, use found in their forum). Results in uncompressed mkv single file of dvd tile. Compress with Avidmux, HandBrake or WinFF from there. Suggested for use to digitize your personal video library (check your countries restrictions). Happy viewing.
I can’t imagine a reason why this would be easier than ffmpeg (or avconv). Of course, I always have a terminal already open.
ffmpeg -i video.mkv -vcodec copy -acodec copy video.mp4
For people looking to convert from mp4 into the much more capable mkv, use the MKVTools https://www.bunkus.org/videotools/mkvtoolnix/ which are designed for this. There is an Ubuntu PPA.
$ mkvmerge -o output.mkv input.mp4 \
subtitle_en.srt subtitle_es.srt …..
With mkvmerge, you can
* correct aspect ratio issues in the header, no re-encoding necessary.
* add/remove audio tracks.
* add/remove subtitles (.sub and .srt)
* append videos
* add images (fan art?, etc)
* add text files (NFO, TXT)
Why would anyone want to convert from the fantastic container, MKV, into the older, lacking, MP4 container? Current android devices support MKV fine as do quality software AND hardware video players. If your device doesn’t, look for a firmware update.
I suppose if you are stuck on a platform without native support, like Windows7, or streaming to a DLNA device without it, like an xbox …