Ensure you're using a media player that supports MKV files. VLC Media Player is highly recommended as it's free, widely available, and supports a vast array of file formats including MKV.
localStorage.setItem('preferred_resolution', '720'); location.reload();
If you have a 720p file downloaded with a 5.1 English audio track but only hear background music or silence (often referred to as missing dialogue), the issue is usually that your media player is trying to send multi-channel audio to a standard stereo (2.0) setup without properly "downmixing" it.