Add, Select, and Remove Multiple Audio Tracks — Managing Multilingual Audio with FFmpeg

A video file can hold several audio tracks, such as Japanese and English audio or a commentary track, and viewers choose one in their player. With FFmpeg you can add, extract and remove these tracks, tag each one with a language code such as jpn or eng, and choose which track plays by default.

Why You Need Multiple Audio Tracks

Common uses:

• Several languages (for example Japanese and English dubs)
• Audio commentary (a second audio track on a film)
• The same audio in two formats (for example AAC and Dolby AC-3)
• Stereo and 5.1ch surround in one file

Inspect Stream Information

First, check how many audio tracks the file has:

ffprobe -v quiet -show_streams -select_streams a input.mkv

Combine Two Audio Files into One Video

ffmpeg -i video.mp4 -i audio_ja.aac -i audio_en.aac \
  -map 0:v:0 \
  -map 1:a:0 \
  -map 2:a:0 \
  -c:v copy -c:a copy \
  output_bilingual.mkv

• 0:v:0 — the video stream of input 0 (video.mp4)
• 1:a:0 — the audio stream of input 1 (audio_ja.aac)
• 2:a:0 — the audio stream of input 2 (audio_en.aac)

Set Language Metadata

ffmpeg -i video.mp4 -i audio_ja.aac -i audio_en.aac \
  -map 0:v:0 \
  -map 1:a:0 \
  -map 2:a:0 \
  -c copy \
  -metadata:s:a:0 language=jpn \
  -metadata:s:a:1 language=eng \
  output.mkv

-metadata:s:a:0 sets metadata on the first audio stream (audio streams are counted from 0). For the language, use three-letter ISO 639-2 codes: jpn, eng, fra, deu and so on.

Set the Default Audio Track

ffmpeg -i input.mkv \
  -map 0 \
  -c copy \
  -disposition:a:0 default \
  -disposition:a:1 0 \
  output.mkv

• -map 0 — keep every stream. Without it, only one audio track is written and the other is dropped without any warning
• -disposition:a:0 default — make the first audio track (a:0) the default
• -disposition:a:1 0 — clear the default flag on the second audio track (a:1)

Add an Audio Track to an Existing Video

ffmpeg -i existing.mp4 -i new_audio.aac \
  -map 0 \
  -map 1:a \
  -c copy \
  output_added.mkv

-map 0 keeps all streams of the existing file, and -map 1:a adds the new audio.

Extract Only a Specific Audio Track

ffmpeg -i input.mkv -map 0:a:1 -c:a copy audio_track2.mka

This copies out the second audio track (index 1, counting from zero) without re-encoding. A .mka (Matroska audio) file takes AAC, AC-3 or any other codec as it is. An .aac file works only when the track is AAC; with an AC-3 track, FFmpeg stops with adts muxer supports only codec aac.

Remove a Specific Audio Track

A -map with a minus sign removes a stream from the selection:

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

-map 0 selects all streams, and -map -0:a:1 then removes the second audio track.

Convert the Codec of an Audio Track

This encodes the first audio track to AAC and copies the second one unchanged:

ffmpeg -i input.mkv \
  -map 0:v -map 0:a:0 -map 0:a:1 \
  -c:v copy \
  -c:a:0 aac -b:a:0 192k \
  -c:a:1 copy \
  output.mkv

-c:a:0 and -c:a:1 set the codec for each audio stream separately.

Caveats with MP4

MP4 handles several language tracks less well than MKV:

To archive files with several audio tracks, use MKV.

Common Problems

The File Became Too Large After Adding an Audio Track

Without -c copy, the video is re-encoded too, and the size changes. With -c copy, the file grows only by the size of the added audio (bitrate × duration). Uncompressed audio such as WAV is added as it is and takes a lot of space, so convert it to AAC or similar first.

The Player Doesn’t Recognize a Specific Audio Track

Keep every track with -map 0 and make the first one the default with -disposition:a:0 default, or check the audio track setting in the player.

List Audio Track Information

ffprobe -v quiet -select_streams a \
  -show_entries stream=index,codec_name,channels:stream_tags=language \
  -of csv input.mkv

Related Articles

• Audio Format Conversion
• Embed Subtitles into Video

FAQ

How do I keep a video that has multiple audio tracks?

Use -map 0 to keep the video and all audio streams (-map 0:a alone drops the video). Add -c copy to skip re-encoding, which is faster and loses no quality.

I want to switch between Japanese and English audio

Tag each audio track with its language, for example -metadata:s:a:0 language=jpn. Players such as VLC, mpv and Plex let you switch audio tracks from a menu even without tags. With tags, they know which track is in which language and can pick the viewer’s preferred one automatically.

Can I use multiple audio tracks in MP4?

Yes. However, some players cannot switch audio tracks, so test on the players your viewers use.

I want to specify which audio plays by default

Use the disposition flags: -map 0 -disposition:a:0 default -disposition:a:1 0. The first audio track (a:0) becomes the default and the second (a:1) does not. Keep -map 0; without it, only one audio track is written.

I want to downmix 5.1ch to 2ch

-ac 2 downmixes to stereo automatically. For finer control, use -af "pan=stereo|FL=...|FR=...".