Replace the deprecated gconfaudiosink with autoaudiosink

The "gconfaudiosink" element doesn't exist in GStreamer 1.x [1]. The
recommended replacement is "autoaudiosink".

Note that MATE doesn't use GConf - it was ported to GSettings and dconf
in MATE 1.6 [2]; and GNOME's official video player, Totem, uses
"autoaudiosink".

[1] gst-plugins-good commit 728732438bbbe81f
https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/commit/728732438bbbe81f

[2] https://mate-desktop.org/blog/2013-04-02-mate-1-6-released/
https://fedoraproject.org/wiki/Features/MATE-Desktop-1.6