Converting AC3 And DTS Audio To AAC In MKV Files

MKV files encoded with AC3 or DTS audio won’t play sound on most iOS and Android devices. These are proprietary formats with licensing restrictions—mobile players either don’t support them or refuse to decode them by default. The video plays fine, but you get silence.

The solution is straightforward: re-encode the audio track to AAC or MP3 while keeping the video stream intact. This takes just a few seconds per file.

Basic Conversion

For a single file, use ffmpeg to convert AC3/DTS audio to AAC:

ffmpeg -i input.mkv -c:v copy -c:a aac -b:a 128k output.mkv

Breaking this down:

-c:v copy — copy the video stream unchanged (no re-encoding)
-c:a aac — encode audio to AAC codec
-b:a 128k — set audio bitrate to 128 kbps (use 192k or 256k for higher quality)

AAC is preferred for mobile devices—it delivers better quality at lower bitrates than MP3. If you need MP3 for older device compatibility:

ffmpeg -i input.mkv -c:v copy -c:a libmp3lame -b:a 192k output.mkv

Check the Audio Codec First

Before converting, verify what codec you’re dealing with:

ffprobe -v error -select_streams a:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 input.mkv

This outputs just the codec name. If it shows ac3 or dts, conversion is needed. For a more detailed view:

ffprobe -v error -select_streams a:0 -show_entries stream=codec_name,codec_long_name,duration -of default=noprint_wrappers=1 input.mkv

Batch Processing Multiple Files

For directories with many MKV files, create a simple bash script:

#!/bin/bash

for file in *.mkv; do
    output="${file%.mkv}_converted.mkv"
    ffmpeg -i "$file" -c:v copy -c:a aac -b:a 128k "$output"
done

Save as convert_audio.sh, make it executable, and run in your media directory:

chmod +x convert_audio.sh
./convert_audio.sh

For more precise control, use find to process files recursively:

find . -name "*.mkv" -exec bash -c '
    for file; do
        output="${file%.mkv}_converted.mkv"
        ffmpeg -i "$file" -c:v copy -c:a aac -b:a 128k "$output"
    done
' _ {} +

Handling Multiple Audio Tracks

If an MKV contains multiple audio tracks, specify which ones to convert:

ffmpeg -i input.mkv -c:v copy -c:a:0 aac -c:a:1 copy output.mkv

This converts the first audio track to AAC and copies the second track unchanged. This is useful when you have both a surround track (AC3) and a stereo track (already compatible).

To copy all audio tracks without re-encoding:

ffmpeg -i input.mkv -c:v copy -c:a copy output.mkv

This is fast but won’t solve AC3/DTS compatibility issues.

To see all audio tracks before processing:

ffprobe -v error -select_streams a -show_entries stream=index,codec_name input.mkv

Parallel Processing for Large Libraries

Audio encoding is CPU-light and typically takes just seconds per file. Speed up batch operations by running multiple ffmpeg instances in parallel using GNU parallel:

find . -name "*.mkv" | parallel ffmpeg -i {} -c:v copy -c:a aac -b:a 128k {.}_converted.mkv

Limit parallel jobs with -j:

find . -name "*.mkv" | parallel -j 4 ffmpeg -i {} -c:v copy -c:a aac -b:a 128k {.}_converted.mkv

Install parallel on Debian/Ubuntu with sudo apt install parallel or on RHEL/CentOS with sudo dnf install parallel.

Verify the Output

After conversion, confirm the audio codec changed:

ffprobe -v error -select_streams a:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 output.mkv

Then test playback on your target device. Modern Android players (MX Player, VLC for Android) and iOS apps handle AAC natively.

If Playback Still Fails

If the file still won’t play after audio conversion, the video codec itself may be incompatible. Check the video codec:

ffprobe -v error -select_streams v:0 -show_entries stream=codec_name input.mkv

If it’s HEVC/H.265 and your device is older, re-encode to H.264:

ffmpeg -i input.mkv -c:v libx264 -preset medium -crf 23 -c:a aac -b:a 128k output.mkv

This is significantly slower than audio-only conversion but ensures maximum device compatibility. Use -preset fast to speed things up at the cost of slightly larger files, or -preset slow for better compression.

Converting AC3 and DTS Audio to AAC in MKV Files