In order to enable the AV features of Gajim it needs gstreamer1.0-gtk3 to be
installed, but this doesn't seem to be in the list of recommended packages.

If gstreamer1.0-gtk3 isn't installed, the create_gtk_widget() function in
/usr/lib/python3/dist-packages/gajim/gtk/gstreamer.py will fail due to
gtkglsink and gtksink not being available and gst-inspect also shows this:

gst-inspect-1.0 gtkglsink
No such element or plugin 'gtkglsink'
gst-inspect-1.0 gtksink
No such element or plugin 'gtksink'

After installing gstreamer1.0-gtk3 , the Audio/Video options become available
and gst-inspect shows info for gtkglsink and gtksink.

I haven't tested the testing or unstable versions of gajim, but I suspect they
might have the same problem as it looks like gstreamer.py references the same
gst modules in gajim 1.9.5.