Fixing AttributeError

Calling AudioFile(audio_dir) where audio_dir is a AudioDirectory
object was causing an AttributeError, since the audio_file variable
was being replaced by a string, before the labels_file variable was
read out from it.

micmon/audio/file.py

@@ -15,8 +15,8 @@ class AudioFile(AudioSource):
                  *args, **kwargs):
         super().__init__(*args, **kwargs)
         if isinstance(audio_file, AudioDirectory):
-            audio_file = audio_file.audio_file
             labels_file = audio_file.labels_file
+            audio_file = audio_file.audio_file
 
         self.audio_file = os.path.abspath(os.path.expanduser(audio_file))