No audio for YouTube videos played within Firefox

My wife could not hear audio today when she was viewing YouTube videos in the Firefox browser. She thought the problem occurred after the automatic update of Windows 10 on her system with the Anniversary Update. The YouTube audio setting was not muted and was turned up to a level where I should have heard sound and I found that I could hear audio for a music file outside of Firefox and I could hear audio for the YouTube video she was trying to view within the Microsoft Edge web browser on the system. And when I went to the CNN website, I was able to listen to the audio associated with a news video on the site.

The Browserspy website's Adobe Flash Information page showed the version of Flash installed on her system was 23.0.0.162.

Browserspy Flash Info

Adobe's Flash Player webpage showed the latest version, 23.0.0.162, was installed. I hit the Alt and H keys to bring up the Firefox help menu, then chose About Firefox, which shows the version of Firefox. The version of Firefox on the system was 49.0.1 and beneath the version was the message "Firefox is up to date."

I typed sndvol in a command prompt window to open the Volume Mixer. You can also get to the Volume Mixer on a Windows 10 system by typng sndvol in the Cortana "Ask me anything window.

Cortana - search for sndvol

The Volume Mixer showed a low olume for Mozilla Firefox, so I increased the volume by dragging the slider bar for Mozilaa Firefox upwards.

Volume Mixer

I could see the green bar in the Mozilla Firefox section of the Voume Mixer moving up and down as the YouTube video played in Firefox, though I couldn't hear any sound.

Volume Mixer - Firefox audio
playing

I muted the volume in Firefox by clicking on the speaker icon in the Mozilla Firefox section of the Volume Mixer.

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Firefox muted in Volume Mixer

When I clicked on the speaker icon again in the Volume Mixer to unmute it, I could then hear sound from the YouTube video play through the speakers.

 

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