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I'm quite new to multimedia things, I'm trying to find a way to make nice screencasts, I've already been able to capture my desktop with ffmpeg:

$ ffmpeg -f alsa -ac 2 -i hw:0,0 -f x11grab -r 30 -s $(xwininfo -root | grep 'geometry' | awk '{print $2;}') -i :0.0 -acodec pcm_s16le -vcodec libx264 -vpre lossless_ultrafast -threads 0 -y out.mkv

It works pretty nice, but I'd like to add music in the background, if I add it with mencoder:

mencoder  -ovc copy -oac mix -audiofile track.mp3 out.mkv -o out.mp4

It delete the video sound, I don't want to replace it, I want to add music to my explanation, any ideas?

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up vote 3 down vote accepted

kdenlive and add a second audio track, then remux

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OMG!, I dint' know kdenlive, I'll definitely check it out!, thx, if anyone else knows how to do it in the console please let me know. – chilicuil Nov 8 '10 at 6:15
sox merge described here: I would split the audio out of the video the sox merge them, then mux it back together with the video – RobotHumans Nov 8 '10 at 6:25
thx a lot aking that's just what I'll do – chilicuil Nov 8 '10 at 9:32

You can install mkvtoolnix;

sudo apt-get install mkvtoolnix

Then use mkvmerge from the installation to mux audio and video into the mkv container from the command line.

There is also a Gui tool, mkvtoolnix-gui that is a front end to mkvtoolnix.

sudo apt-get install mkvtoolnix-gui

Example to mux video file called, input-video.mkv with a sound file called soundfile.mp3, creating a new muxed mkv file called output-with-sound.mkv, do the following in the directory where you have the video without audio and the audio file:

mkvmerge -o output-with-sound.mkv -A input-video.mkv soundfile.mp3

More examples including synchronization here:

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thx a lot sabacon!, I didn't know about mkvmerge, I'll use it to mux back the mixed audio – chilicuil Nov 8 '10 at 9:33
correct install command is sudo apt-get install mkvtoolnix-gui – augusto Feb 13 at 12:21
@augusto Thanks a lot, I did not notice the extra hyphen, and to know this thing was there for so long. – Sabacon Feb 13 at 17:59

I'm not sure why mencoder didn't work for you, but you can also do the same thing with ffmpeg. You just need to use two inputs to ffmpeg.

 ffmpeg -i track.mp3 -i out.mkv -vcodec copy -acodec libfaac -ab 192k final.mkv

I didn't try it, but I don't think you can do -acodec copy, especially since you're using two different codecs. Just pick whatever you want in the output and add that in, its really fast compared to the video.

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