If only audio plays, use a converter like Handbrake to change the container to MP4, or update your media codecs.
I can give you a specific solution, such as recommending a converter or the best player to use. download large mkv files with php script - Stack Overflow
If the file freezes or cannot skip ahead, utilize Emby's advice on managing MKV issues.
Likely a file-sharing service or messaging app like Telegram.
Click the download icon, which will save the file to your "Downloads" folder.
.mkv (Matroska) is a flexible container, often requiring specific players, rather than standard default players.
VLC Media Player (best for Windows/Mac/Linux). Alternative: DivX Player.