Thanks very much for your help.

Before I could do anything about changing /etc/asound.conf,
the sound suddenly re-appeared in Iceweasel. I am reluctant to
change things now, but if the sound goes again, I will try your
suggestion for /etc/asound.conf

Thanks again for your help.

Regards,

Mike Bailey


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30. Oct 2017 23:21 by pg_oxl@oxl.for.sabi.co.UK:

[ ... ]
That might be instead (and just in '/etc/asound.conf'):
------------------------------------------------------------------------
pcm.pulse { type pulse; }
ctl.pulse { type pulse; }

pcm.!default { type pulse; fallback "sysdefault"; }
ctl.!default { type pulse; fallback "sysdefault"; }
------------------------------------------------------------------------

[ ... ]
% cat /etc/asound.conf
pcm.!default {
type hw
card 0
}
ctl.!default {
type hw
card 1
}

[ ... ]
card 0: PCH [HDA Intel PCH], device 0: ALC887-VD Analog [ALC887-VD Analog]
Subdevices: 0/1
Subdevice #0: subdevice #0

That is certainly not the default configuration, and also it
overrides it, which is unfortunate as that chipset, like most,
does not have hardware mixing and that '/etc/asound.conf'
disables software mixing.
Am I right in thinking that the contents of /etc/asound.conf
indicate that pulseaudio is not being used, but ALSA alone is
used for the sound ?

Yes.
In addition, the YouTube sound has now suddenly re-appeared
without any obvious reason. This indicates a pattern of sound
suddenly disappearing without obvious reason and then suddenly
re-appearing days or perhaps weeks later. Could this problem
be due to certain programs suddenly stopping or restarting the
sound ?

No. Almost surely given the lack of hardware mixing and software
mixing having been disabled it means that some other program was
using the soundcard.

Any good reason why the suggested 'asound.conf' was not tested?

Maybe there have been good reasons why the additional steps in
the suggested troublesheeting guide related to software sharing
were not followed...
http://www.sabi.co.uk/Notes/linuxSoundALSA.html#share

But fortunately nowadays the 'sysdefault' virtual device
(listable with 'aplay -L') usually has sharing enabled by
default.