So I have successfully used Handbrake (version 1.1.2) to convert many large MKV files into smaller, iOS and macOS compatible video files I can play on my iPhone and MacBook Air for a while without really having any issues until this one specific movie.

When I convert it with Handbrake using default “General” settings of seemingly any type—Fast, Very Fast, HQ or Super HQ—the audio downmixing from 5.1 audio results in weird stereo with essentially all of the audio going to the right channel and the left channel seems to have some channel that is just—for lack of a better term—background remnants. Just enough sound to not be silent but utterly no music or dialogue or anything is really audible.

I can play it on VLC (version 3.0.6) on my MacBook Air and I get the same odd sound through my speakers; heavy right barely anything left. But if I set the audio of VLC to “Headphones”… Wow! It becomes a normal stereo downmix.

What is happening and what can I do to properly downmix this? I hope I can do this in Handbrake, but if not I assume I would have to extract the AC3 out of the MKV, downmix it in some other tool and then toss it back into the MKV?

Details of the audio track here using ffmpeg -i:

Stream #0:1(eng): Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s (default) (forced)

Metadata:

  title           : English

  BPS-eng         : 384000

  DURATION-eng    : 01:36:42.624000000

  NUMBER_OF_FRAMES-eng: 181332

  NUMBER_OF_BYTES-eng: 278525952

Details of the audio track here using mkvmerge --identify is:

Track ID 0: video (MPEG-4p10/AVC/h.264)

Track ID 1: audio (AC-3)

Details of the audio track here using mkvinfo is:

| + Track

|  + Track number: 2 (track ID for mkvmerge & mkvextract: 1)

|  + Track UID: 3241101187

|  + Track type: audio

|  + Forced track flag: 1

|  + Codec ID: A_AC3

|  + Default duration: 00:00:00.032000000 (31.250 frames/fields per second for a video track)

|  + Name: English

|  + Audio track

|   + Sampling frequency: 48000

|   + Channels: 6

Details of the audio track here using exiftool is:

Track Number                    : 2

Track Type                      : Audio

Track Forced                    : Yes

Audio Codec ID                  : A_AC3

Default Duration                : 32 ms

Track Name                      : English

Audio Sample Rate               : 48000

Audio Channels                  : 6

Once you know the channel mapping, you can just use the channelmap filter, skipping the intermediate files.

ffmpeg -i my_movie.mkv -c:v copy -c:s copy -af "channelmap=0|2|1|3|4|5" -c:a ac3 out.mkv

The “Center” channel was actually “Right” and visa-versa.

Swapping them results in a properly balanced, normal sounding AC3 file.

I discovered this based on Gyan’s comment, I decided I can fix this only by actually breaking out individual mono tracks from the AC3, listening to each, and then reconstructing the AC3 to fix the audio layout. Thus, this discovery and the solution to this mini mess.

That said, all of the steps I outline below worked for me, but if anyone knows a cleaner way to do this in one command, please feel free to post as an answer. I will give you the glory since this is literally the most elaborate FFmpeg and MKV manipulation I have ever done, so it was a learning process.

First, I used mkvmerge to identify which tracks are which in my_movie.mkv:

mkvmerge --identify my_movie.mkv

Knowing that I extracted the AC3 file using FFmpeg to a file named my_movie.ac3:

ffmpeg -i my_movie.mkv -map 0:1 -c:a copy my_movie.ac3

Then I took that my_movie.ac3 and exploded it into 6 distinct mono tracks:

ffmpeg -i my_movie.ac3 -map_channel 0.0.0 front_left.wav -map_channel 0.0.2 front_right.wav -map_channel 0.0.1 front_center.wav -map_channel 0.0.3 back_left.wav -map_channel 0.0.4 back_right.wav -map_channel 0.0.5 lfe.wav

Then I just recombined those mono tracks into a new AC3 file called my_movie_fixed.ac3:

ffmpeg -i front_left.wav -i front_right.wav -i front_center.wav -i lfe.wav -i back_left.wav -i back_right.wav -filter_complex "[0:a][1:a][2:a][3:a][4:a][5:a]amerge=inputs=6[a]" -map "[a]" my_movie_fixed.ac3

Then I added the newly created my_movie_fixed.ac3 to my_movie.mkv and called the resulting file my_movie_fixed.mkv:

ffmpeg -i my_movie.mkv -i my_movie_fixed.ac3 -map 0 -map 1 -c copy my_movie_fixed.mkv

It worked! Sound is great via normal “Stereo” playback in VLC as well as when the my_movie_fixed.mkv was converted via Handbrake.

That said, I also adjusted the metadata specifics for the track and set it as a default audio track using mkvpropedit like this:

mkvpropedit my_movie_fixed.mkv --edit track:a2 --set name=English --set language=en