MKV names the container
Matroska video files commonly use the .mkv extension. A container brings tracks together: video, audio, subtitles and other information. The video compression format inside it is a separate decision. Reading “MKV” alone cannot establish sharpness, language or compatibility with your television.
MP4 is another container, not the opposite of HD
An MP4 can contain high-definition video, and an MKV can contain a low-resolution picture. What matters for playback is the combination of container, video codec, audio codec and device support. Changing the letters at the end of a filename does not convert those contents.
When the picture works but audio does not
A player may decode the video track but not the selected sound format. Check whether another audio track exists, whether the device supports that codec and whether the receiver or television is handling the sound. Raising volume will not fix an unsupported codec.
Remuxing and re-encoding are different
Remuxing repackages compatible tracks in another container. Re-encoding recompresses a track and can change its size and quality. Neither operation creates missing visual detail or a new Hindi dub. Use the file-size calculator for a simple duration-and-data comparison, and the quality guide for picture choices.