Previously we would only read the default sink and default source when
the connection to PulseAudio succeded. This worked because all VolumeMenu
users where initialized synchronously during shell load.
With the recent session mode changes though, the lock screen menu is
created on demand, and when it loads PA is already connected, so
it doesn't update the sliders.
https://bugzilla.gnome.org/show_bug.cgi?id=683156