Sounds like a problem with the downloaded. Video and audio are sent separately and combined by the downloader.

Try this; https://www.dvdvideosoft.com/

This Is ONLY A PC App https://www.dvdvideosoft.com/products/dvd/Free-YouTube-Download.htm Use this YouTube downloader to get single videos and complete playlists, show list, charts and user channels from YouTube. Convert to AVI, MP3, WMV.

MKV = Matroska Multimedia Container: https://en.wikipedia.org/wiki/Matroska

Firefox doesn't support this container format although it is similar to WebM (but not the same). I don't know why you would get this MKV format and not a more common type.

• https://developer.mozilla.org/Media_formats_supported_by_the_audio_and_video_elements

Do you otherwise have a problem with playing media files?

You can open the YouTube test page to check support in Firefox for playing HTML5 media.

• https://www.youtube.com/html5

i think i will stick with google chrome in case of downloading a video from youtube

thanks everyone for help

cor-el said

MKV = Matroska Multimedia Container: https://en.wikipedia.org/wiki/Matroska Firefox doesn't support this container format although it is similar to WebM (but not the same). I don't know why you would get this MKV format and not a more common type.

• https://developer.mozilla.org/Media_formats_supported_by_the_audio_and_video_elements

Do you otherwise have a problem with playing media files? You can open the YouTube test page to check support in Firefox for playing HTML5 media.

• https://www.youtube.com/html5

no i don't have any problems with playing media files

but when i download the video with IDM from youtube i found the mkv format and the video has no sound

i think there is something with the new version of firefox quantum