If the sink has properties named volume and mute, we have no idea about their
meaning. The streamvolume interface standardizes the meaning.
In the case of osxaudiosink for example, the current volume property has a
range of 0.0 to 1.0, but we need 0.0 to 10.0 or similar. Also osxaudiosink
has no mute property. As such, the volume element should be used here instead.
https://bugzilla.gnome.org/show_bug.cgi?id=752156