Fix the map index of externel audio

MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs

@@ -2273,7 +2273,10 @@ namespace MediaBrowser.Controller.MediaEncoding
                 int audioStreamIndex = FindIndex(state.MediaSource.MediaStreams, state.AudioStream);
                 if (state.AudioStream.IsExternal)
                 {
-                    bool hasExternalGraphicsSubs = state.SubtitleStream != null && state.SubtitleStream.IsExternal && !state.SubtitleStream.IsTextSubtitleStream;
+                    bool hasExternalGraphicsSubs = state.SubtitleStream != null
+                        && state.SubtitleDeliveryMethod == SubtitleDeliveryMethod.Encode
+                        && state.SubtitleStream.IsExternal
+                        && !state.SubtitleStream.IsTextSubtitleStream;
                     int externalAudioMapIndex = hasExternalGraphicsSubs ? 2 : 1;
 
                     args += string.Format(