fix ffmpeg metadatapp

youtube_dl/postprocessor/ffmpeg.py

@@ -464,7 +464,11 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
         filename = info['filepath']
         temp_filename = prepend_extension(filename, 'temp')
 
-        options = ['-c', 'copy']
+        if info['ext'] == u'm4a':
+            options = ['-vn', '-acodec', 'copy']
+        else:
+            options = ['-c', 'copy']
+
         for (name, value) in metadata.items():
             options.extend(['-metadata', '%s=%s' % (name, value)])