diff --git a/mopidy/backend/__init__.py b/mopidy/backend/__init__.py
index 68aad778..d6474c43 100644
--- a/mopidy/backend/__init__.py
+++ b/mopidy/backend/__init__.py
@@ -12,6 +12,11 @@ class Backend(object):
     :exc:`~mopidy.exceptions.BackendError` with a descriptive error message.
     This will make Mopidy print the error message and exit so that the user can
     fix the issue.
+
+    :param config: the entire Mopidy configuration
+    :type config: dict
+    :param audio: actor proxy for the audio subsystem
+    :type audio: :class:`pykka.ActorProxy` for :class:`mopidy.audio.Audio`
     """
 
     #: Actor proxy to an instance of :class:`mopidy.audio.Audio`.
diff --git a/mopidy/mixer.py b/mopidy/mixer.py
index 01216186..6b15efb2 100644
--- a/mopidy/mixer.py
+++ b/mopidy/mixer.py
@@ -8,6 +8,11 @@ class Mixer(object):
     :exc:`~mopidy.exceptions.MixerError` with a descriptive error message. This
     will make Mopidy print the error message and exit so that the user can fix
     the issue.
+
+    :param config: the entire Mopidy configuration
+    :type config: dict
+    :param audio: actor proxy for the audio subsystem
+    :type audio: :class:`pykka.ActorProxy` for :class:`mopidy.audio.Audio`
     """
 
     name = None