You could also give us an example link to the YouTube video and let us here see if we have the same problem. You could also use FireFox and any of their video downloading add-ons which I know work. You might try another video downloader Chrome add-on, or try installing and using using 'youtube dl' or youtube dl gui' to download videos from YouTube. So, It sounds like this 'idm' is not downloading the file(s) properly from YouTube using Chrome, or your method of doing this is somehow not working, or the file is not really a '.mkv' file. I do not know what 'idm' is? Nor do I see why Chrome and or this 'idm' would make a difference in whether VLC can play '.mkv' files plus, you just said that VLC can indeed play other '.mkv' files. for these files vlc playing video but no audio. mkv files which downloaded using chrome and idm. and one more thing some other mkv files are able to play with vlc, this particular file I downloaded from youtube by using idm via google chrome.

Ya i tried these thing, when playing this file with different multimedia player in Linux it is not playing. My VLC plays almost anything I ask it to including '.mkv' files. Sudo add-apt-repository ppa:videolan/stable-daily

You can also make sure that VLC is current by using its stable PPA. The other user was trying to tell you to try another multimedia player in Linux to see if it would play the '.mkv' files, like 'smplayer'. Try going into your Software Manager or Synaptic Package Manager (SPM), look up VLC and 'mkv' and install anything that seems relevant, like all VLC plugins, and install ' ubuntu-restricted-extras', 'mplayer2' stuff, & 'smplayer'.

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