GStreamer 1.0 does not have mixer interface. MATE folks create small GObject library to replace it:)
I rely on xfce4-mixer as I really don't like the Indicator plugin and run sound primarily through Jack into ALSA and not PA at all, so the standard mixer interface is of very little help. The fact it has been dropped from Ubuntu 16.04 due to the fact development has stopped as no mixer function in GStreamer1.0 has forced me to install it from an older repository (even though 16.04 still has GSteamer0.1 in its repos so I see no need for the mixer to have been dropped yet.) Therefore continuing the project with a library which will have continued support into the future is something I would really like to see.
Closing old bugs. Mixer is being rewritten