You can't do that when playing the file back via the web interface. We can only reliably play MP4 videos on browsers and most of them don't support multi-track MP4 files. So when we convert MKV files to MP4 for web playback, we only use the first audio track during conversion. Which means, in web playback, you can only play the first audio track in the original file. 

You can, however, use the Download or Stream link options to play the original video with a desktop video player to have access to the original audio tracks.

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