Command to extract audio from video without conversion

ffmpeg -i video.mp4 -vn -acodec copy audio.aac

Here’s a short explanation on what every parameter does:

• -i option specifies the input file.
• -vn option is used to skip the video part.
• -acodec copy will copy the audio stream keeping the original codec.

As a complementary answer, with a command that doesn't need the specific name of the file.

For a video containing aac audio:

ffmpeg -i "$0" -map 0:1 -c:a copy "${0%%.*}".m4a

To use in a context menu command, a form like this works:

bash -c 'ffmpeg -i "$0" -map 0:1 -c:a copy "${0%%.*}".m4a' %f 

Replace with corresponding audio format output for other video format input.

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Initial source here.