Matroska is a multimedia container widely know for it's video format (mkv). Lesser known are the audio (mka) and subtitles (mks) formats. Also the stereoscopic/3D format (mk3d). Aside from being open and royalty-free, why are these formats interesting?

Tools

MKVToolNix is a CLI (optional GUI) tool to work with Matroska files. It works with mkv and mka, which are the two formats I tested. It's a single tool to work with both audio and video formats.

Features

Matroska is a do-it-all "universal" container that does many things well. The most interesting being:

1. Chapters. For an audio file that means an entire album or audiobook can be contained in a single large mka file splitted by chapters, similar to m4b files. It's easy to make such a file in mkvtoolnix, would save me a lot of time from picking every ogg chapter file in librivox recordings.
2. Multiple streams. A single file can hold different codecs (e.g. vorbis, opus and MP3) or different bitrates and they can be muxed from a supported player. Video players commonly support it whereas I am not aware of audio players with this feature.
3. Subtitle embedding. Closed captions are popular in mkv, but mka could also hold an entire timestamped audiobook or music lyrics inside. Haven't tested it though, and I think there's no player that supports such scheme.

See the FAQ for more info.

So, what about MKS?

I couldn't find documentation about it, or even sample files. Maybe you can help.