You need to set AudioInputPort to true for the writer, not the reader. From the help:
"Write audio data, specified as false or true. Use this property to control whether the object writes audio samples to the video file. Set this value to true to write audio data. To write audio and video to a file, you must use the .avi format ."
So it appears you can only add audio for an .avi format, not an .mp4 format video.