Many people ask if they can convert the audio (usually DTS, DTS-HD or Dolby TrueHD) of their .mkv file to AC3 audio (Dolby Digital) without extracting the audio, converting the audio to AC3 and then muxing the audio back to .mkv. There is a much simpler way by using FFmpeg. You can download compiled FFmpeg from ffmpeg.zeranoe.com (Compiled builds for Windows). Selecting the Static build is recommended.
Using the following command, all the tracks (video, subtitles, chapters etc.) are copied to a new .mkv file unaltered except the audio track, which is converted to AC3. Please note that FFmpeg supports only 5.1 channels for AC3 conversion at the moment.
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