audio: Make sure software mixer emits mute events.

Turns out that gobject.GObject.set_property does not have a return value.

docs/changelog.rst

@@ -16,6 +16,9 @@ Bug fix release.
   out this can cause distortions in certain cases. Give this an other go with
   a more generous buffer size. (Fixes: :issue:`1147`)
 
+- Audio: Make sure mute events get emitted by software mixer.
+  (Fixes: :issue:`1146`)
+
 
 v1.0.1 (2015-04-23)
 ===================

mopidy/audio/actor.py

@@ -194,16 +194,14 @@ class SoftwareMixer(object):
 
     def set_volume(self, volume):
         self._element.set_property('volume', volume / 100.0)
-        self._mixer.trigger_volume_changed(volume)
+        self._mixer.trigger_volume_changed(self.get_volume())
 
     def get_mute(self):
         return self._element.get_property('mute')
 
     def set_mute(self, mute):
-        result = self._element.set_property('mute', bool(mute))
-        if result:
-            self._mixer.trigger_mute_changed(bool(mute))
-        return result
+        self._element.set_property('mute', bool(mute))
+        self._mixer.trigger_mute_changed(self.get_mute())
 
 
 class _Handler(object):