the error occurs because the temp file doesn't exist. That is probably because something didn't happen in the ffmpeg muxing.
Change line 96 to add a print to the beginning of the statement as in:
Code:
print subprocess.Popen([ffmpeg, '-i', video_temp_name, '-i', audio_temp_name, '-acodec', 'copy', '-vcodec', 'copy', muxed_temp_name], stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()
Post the results