According to Rodrigo Rivas:
One last idea. Maybe the gnome-settings-daemon is playing dumb with your sound. I think you can disable the sound plugin of g-s-d using dconf (org.gnome.settings-daemon.plugins.sound.active).
I tried dconf write org/gnome/settings-daemon/plugins/sound/active false
I don't know if that will affect also to the GDM greeter, but it is worth trying it. It had no effect, either in the greeter or in GNOME itself.
As a last resort you could also try renaming "/usr/lib/gnome-settings-daemon-3.0/libsound.so" and see what happens.
Strangely, this also has no effect at all. Once GDM starts, the master volume is still zeroed out and muted. I say zeroed out and muted because I must run alsamixer, turn up the master volume and then unmute it in order to get the sound working again, although sudo systemctl start alsa-restore also does work, since the volume was previously saved using sudo systemctl start alsa-store while the volume was at the proper level. At this point, I am totally stumped. The computer I had that died used a SoundBlaster Live Value, and although the sound started out muted, restoring the alsa volumes always worked as expected. However, on this machine with the Intel onboard sound, nothing seems to keep the volumes from muting whenever GNOMe and GDM start. ~Kyle