Whenever I buy a Bluray, I convert it to an mp4 using a combination of makemkv and ffmpeg. This allows me to store my films on a single large hard drive without using up shelf space I don't have. I then discard the original disc, to stay legal.
However, because ffmpeg has a very poor aac encoder, I usually convert the audio to ALAC first, and then use Apple's built-in afconvert utility to convert said ALAC to aac audio.
afconvert DayOfTheDocotor-alac.m4a -d aac -b 256000 -f m4af DayOfTheDoctor.m4a
I then mux the video and audio into a single mp4 with ffmpeg.
This method works great for stereo sound, but whenever I use afconvert on a 5.1 audio track, it gives me the following warning message:
*** warning: Transcoding multichannel audio may not handle channel layouts correctly (0)
It then proceeds to convert the audio, and as far as I can tell on my stereo speakers, everything is working fine. I don't actually have a 5.1 sound system at the moment, but I might get one someday, and when I do, I want to be able to play my existing movies.
So my question is, what does this warning message mean, and is afconvert properly transcoding my 5.1 audio?