GVC added additional checks that break where we were using a NULL value instead
of a GvcMixerControl. Solution is to pass this object where appropriate.

(gnome-control-center:833): Gvc-CRITICAL **: 11:09:33.818: gvc_mixer_control_get_vol_max_norm: assertion 'GVC_IS_MIXER_CONTROL (control)' failed

Fixes #636