User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:55.0) Gecko/20100101 Firefox/55.0 Build ID: 20170814072924 Steps to reproduce: Install Firefox 55 official release, Firefox Developer Edition 56.0b3, or Firefox Beta 56.0b3, create new profiles with no addons. Go to http://camendesign.com/code/video_for_everybody/test.html or any video on Vimeo.com such as https://vimeo.com/230046783, and go to https://www.youtube.com/html5?gl=CA to see browser test results. Actual results: The video on Vimeo auto-play but no sound comes through. On the http://camendesign.com/code/video_for_everybody/test.html test video, the volume button is crossed out and there is no sound. On live Youtube videos (ie. https://www.youtube.com/watch?v=ArwVHSydxIw), it says "Your browser does not recognize any of the video formats available" and neither video nor audio is played. On non-live Youtube videos (ie. https://www.youtube.com/watch?v=-5xdTlgaaaw), audio and video works perfectly fine so there is no problem with normal Youtube videos. On the Youtube HTML5 browser test page (https://www.youtube.com/html5?gl=CA), it says all are supported except for H.264. Expected results: Expected result is that all videos on Vimeo have audio, and that Youtube live streams actually play.
Component: Untriaged → Audio/Video: Playback
Product: Firefox → Core
You appear to be using Windows N or KN. There should be a drop down at the top of the page directing you to https://support.mozilla.org/sw/kb/fix-video-audio-problems-firefox-windows
I am using Windows 7 Ultimate x64 SP1, so all Media Foundations codecs should be installed.
(In reply to Stan from comment #2) > I am using Windows 7 Ultimate x64 SP1, so all Media Foundations codecs > should be installed. Have you ever used the K-Lite Codec Tweak Tool?
Yes, I have used the K-Lite Codec Tweak Tool in the past to override my Windows Media Center - Microsoft DTV decoders with FFDShow (so that I can mirror front stereo channels to my rear speakers in Windows Media Center when watching over-the-air TV channels that broadcast in Dolby 5.1). I have narrowed this down as the source of the problem. Using the Codec Tweak Tool, I reset all my codecs, rebooted, reset again, rebooted, reset some Firefox settings to default, and now I have audio on those videos that I didn't have audio on before. It was strange that YouTube and select other websites had audio that worked fine, but I suppose they automatically changed the audio format to something an active audio decoder could decode? However, please note that Chrome was able to play the audio on those same websites at the same time I was not able to play the audio in Firefox. Something in the Firefox implementation of audio decoding differs from the audio decoding of Chrome.
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