Comment 13 for bug 1403938

A workaround for those who are getting their ears blasted:
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For those who haven't yet lost patience with FireFox, having found out that this CubebUtils thing
has got into Firefox through this thread. Weirdly, I only noticed it in 47.0 and had no issues prior.

On https://people.mozilla.org/~bgirard/doxygen/media/CubebUtils_8cpp.html a setting called "media.volume_scale" has been added, default value 1.0.

1) Type "about:config" into a tab.
2) Click the "I'll be careful" box on the "Here be dragons..." screen.
3) Search for "volume" and -- as of writing -- the only setting you'll find is the relevant one.
4) Reduce the number; see below for some hints.
5) Play music or a video with pavucontrol (PulseAudio Volume Control) to see where the volume level gets to: tweak the number again if needed.

The bugger will still jump up to a set volume level regardless of what you set it to in pavucontrol, but tweaking this setting sets a ceiling of sorts.

By way of illustration: The volume kept jumping to 89% and blowing my eardrums before this tweak with media.volume_scale at 1.0. I set it to 0.1 and it seems stable around 46%; different videos go between 41% and 49% at times. See what numbers your system throws at you.

If this is set to 0, no sound will play. If you don't get any sound at all, check this setting.