Comment 4 for bug 1313813

On further investigation. Looks similar to Bug #1026331 (duplicate of #878986). Which, according to the comments on the latter is a Pulseaudio issue.

Fixed (or rather triaged) by using
amixer -D pulse set Master toggle
(which worked for me)
or using
pactl set-sink-mute <default_sink_name> 0
(which wouldn't recognise the sink name)