hello christan,

Unmute Firefox and plugins

1. Open Firefox and play content that should play sound, such as this Firefox video or a Flash video with sound.
2. Click the volume icon in the Windows taskbar.
3. Click Mixer. The Volume Mixer window will appear.
4. Make sure the sliders for Mozilla Firefox and Plugin Container for Firefox (for plugin content such as Adobe Flash) are not muted or at the bottom.

Check your audio device Make sure that the audio device (such as speakers or headphones) are turned on and connected to the computer properly.

Other solutions Check out whether other applications on your computer can play sounds. If they can't:

Follow Microsoft's Fix sound problems tutorial. If Firefox is the only application that you can't hear sound in:

Try the solutions in Fix common audio and video issues.

Share this article: http://mzl.la/1xKrBJ7

for better understand I attached image

I hope resolve your problem then reply back to me on reply and comment section,

Thank you for asked question in Mozilla .

Regards, Arman

https://support.mozilla.org/en-US/kb/what-to-do-if-firefox-wont-play-any-sounds https://docs.microsoft.com/en-us/deployedge/microsoft-edge-relnote-stable-channel

Arman and Pallavi thanks for your replies. Your suggestions have not solve the Firefox sound issue. It apperas to be more a Firefox issue than Win10 issue. Reason for saying that is that VLC works fine. Youtube audio/music works fine in Microsoft Edge but not in Firefox - it was working prior to Firefox upgrade to 77.0.1 from 77.0