Have you tried saving the video first and doing it locally? Maybe that's the problem, or your version or build of ffmpeg. I had no problem with the following line and version N-60710-g61d5970.
Code:
ffmpeg -i shaykh.mp4 -vn -acodec copy shaykh.m4a
That is a completely lossless operation which simply puts the original audio stream in a new container, in this case M4A, so there is no loss in quality.