Pass backend reference to the mixer constructor, just like for the other controllers

mopidy/backends/base/__init__.py

@@ -36,7 +36,7 @@ class BaseBackend(object):
         if mixer is not None:
             self.mixer = mixer
         else:
-            self.mixer = get_class(settings.MIXER)()
+            self.mixer = get_class(settings.MIXER)(self)
 
     #: A :class:`multiprocessing.Queue` which can be used by e.g. library
     #: callbacks executing in other threads to send messages to the core

mopidy/mixers/__init__.py

@@ -1,6 +1,11 @@
 class BaseMixer(object):
-    def __init__(self, *args, **kwargs):
-        pass
+    """
+    :param backend: a backend instance
+    :type mixer: :class:`mopidy.backends.base.BaseBackend`
+    """
+
+    def __init__(self, backend, *args, **kwargs):
+        self.backend = backend
 
     @property
     def volume(self):