This is gonna be more readable in two lines.

I think you could just set these as bindings rather than programmatically doing it onClicked

How would I do that?

You could also add a if (volume == 0) { return "audio-volume-muted" } and save the muted check below :)

Button {
    ...
    icon.name: muted ? "audio-volume-muted" : soundState(root.pluginInterface.volume)
    ...
}

as well as

Slider {
    ...
    enabled: !muted
    ...
}

and then just keep

onClicked: {
    muted = !muted
    root.pluginInterface.volume = muted ? 0 : volumeUnmuted
}

Nope, as 0 doesn't always mean muted

The setting icon name as property in button also fails, maybe qml doesnt check as isnt direct variable

If you want the property to update instead of just setting the value, use Qt.binding().
http://doc.qt.io/qt-5/qtqml-syntax-propertybinding.html

It could also make sense to use visualPosition.

Tried that, doesn't work

Both?
Check dbusinterfaces.cpp:128 and make sure the signal is being emitted?

How about naming it toggleMute()?