FF vid    FF audio    Edge vid    Edge audio
av1-opus-sita.webm          Works     Works       Works       Works
vp9-vorbis-spring.webm      Works     Works       Works       Works
vp8-vorbis-sintel.webm      Works     Works       Works       Works
avc-aac-nerdist-friends.mp4 Works     MISSING     Works       Works
avc-aac-big-buck-bunny.m4v  Works     MISSING     Works       Works
avc-mp3-sita.mov            Works     Works       Works       Works

                   FF audio   Edge audio
audio-sample.mp3    Works      Works
audio-sample.weba   Works      Works
audio-sample.ogg    Works      Works
audio-sample.opus   Works      Works
audio-sample.flac   Works      Works
audio-sample.wav    Works      Works
audio-sample.m4a    FAILS      Works
ac192.mp3           Works      Works
spoonradio-hd.aac   FAILS      Works
audio.opus          Works      Works

I've recently done a clean reinstall of Windows 11 and have noticed that, only for firefox (134.0.2, build ID 20250120135430), a few videos playback without any audio, yet the same videos play ''with'' audio when using edge as a comparison. Most (if not all) of these videos did play with audio in firefox prior to this reinstall. Firstly, I'm pretty sure this is not a windows sound mixer issue since many videos on the same page and tab play back with audio just fine in firefox. The same goes for it not relating to the firefox setting 'Speaker selection', under (burger) > settings > privacy & security > Permissions because videos of different formats (or using different players) on the same page play back audio fine. The tab is not muted. Again, some videos on the same browser tab play audio fine; some of a particular format or player only play the video, while missing the audio (and where Edge plays the same video with audio). Another common and potentially relevant issue I've seen is that, this being a 'Windows 11 N" install, it was initially missing the settings > system > optional features > Media feature pack, but that is now something I've successfully added. I should add that windows did report problems when I initially tried to add the media feature pack but, after a very long delay, it eventually appeared to succeed when I ran a couple of DISM incantations that I think ended with - "dism /online /add-capability /capabilityname:Media.MediaFeaturePack~~~~0.0.1.0" I have the 'Play DRM-controlled content' setting enabled. Comparing the firefox vs edge video playback on this video test page - https://tools.woolyss.com/html5-audio-video-tester/ I get the following results (don't know how to get it formatted/aligned correctly, sorry) - FF vid FF audio Edge vid Edge audio av1-opus-sita.webm Works Works Works Works vp9-vorbis-spring.webm Works Works Works Works vp8-vorbis-sintel.webm Works Works Works Works avc-aac-nerdist-friends.mp4 Works '''MISSING''' Works Works avc-aac-big-buck-bunny.m4v Works '''MISSING''' Works Works avc-mp3-sita.mov Works Works Works Works And perhaps also relevant is that there's also a difference in the audio-only samples further down the table on the same page - FF audio Edge audio audio-sample.mp3 Works Works audio-sample.weba Works Works audio-sample.ogg Works Works audio-sample.opus Works Works audio-sample.flac Works Works audio-sample.wav Works Works audio-sample.m4a '''FAILS''' Works ac192.mp3 Works Works spoonradio-hd.aac '''FAILS''' Works audio.opus Works Works Also, every 'v.redd.it' / reddit-video-player video plays the video fine but has no sound when using firefox but plays both sound and video fine with Edge. Here's one for example - https://old.reddit.com/r/funny/comments/1i7y1nr/rock_paper_scissors/ I've seen lots of similar (but not identical) issues on firefox video with no audio problems with suggestions a few things to try. In addition to what I've already mentioned above, I've tried the following without success - - Restarting windows - Restarting firefox - Restarting firefox with addons/extensions disabled - 'about:support' > Refresh firefox - 'about:support' > Clear startup cache - (hamburger) > more tools > task manager > (click the little profile icon for the page playing the video) - I've checked the settings suggested here: https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues#w_check-some-firefox-settings I realise it may be possible to side-step this problem by flailing around with more drastic work-arounds like trying a clean re-install of windows or a full uninstall and clean of firefox, but that doesn't really help figure out the problem, which leaves the possiblity it could easily happen again to me or others when it might be something that can be more properly fixed or guarded against in firefox itself, so I hope someone can suggest what might be done to home in on identifying the cause of the problem and so learn whether a more precise fix can be employed.