WebkitGTK-based browsers: System volume suddenly maxed out when playing audio or video

[Alek Zikon]

Everytime audio or video starts playing on WebkitGTK-based browsers (epiphany, 
next), the system volume is maxed out. This happens when you start the audio or 
video by clicking on the play button and also when audio or videos are played 
automatically (in a playlist, or ad videos, for example).

This issue has been reported before upstream, but epiphany people say the 
source of the problem is in pulsaudio defaults on distros 
(https://gitlab.gnome.org/GNOME/epiphany/issues/73):

  Thanks for reporting this issue. You'll need to ask Debian to disable
  PulseAudio's flat volumes feature, as is done by all other major
  distributions (Ubuntu, Arch, Fedora, openSUSE, probably more),
  since we're not going to make any changes here.

I found that there is a related pulsaudio bug reported on Guix 
(https://issues.guix.gnu.org/issue/38172). Unfortunately, this issue is still 
open.

Epiphany people say you, as a user, can work around the issue by setting 
"flat-volumes = no in your /etc/pulse/daemon.conf." What's the correct way to 
this on the Guix System?


I'm using this software:

epiphany 3.30.4
WebKitGTK+ 2.26.1
GNOME 3.30.2

$ guix describe
Generation 16   Jan 03 2020 14:36:37    (current)
  guix 7158fe4
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 7158fe4ded47a599ceb8d556132ba83fcc686962