Thanks for your feedback.
The problem is the following: I tagged MKV-files with MP3Tag. Usually, like in all FLAC-files, the tags are shown in MC. But this doesn't work with MKV-files; as I've written, in the Tag Editor, the Album field is empty.
So I tried the other way round: I tagged the field Album in MC. But MC writes the Album-title only in it's own JRSidecar.xml-file, not in the MKV itself.
With the Composer-field there is an additional problem: Since it's a predefined field, in the Library-Manager I can't change the field-option "Flags" from "Audio" to "Audio; Video" (the same with other fields: the possibility to change the "Flags" afterwards would be requiered). So, even if I want to make an addition tagging in MC to use the JRSidecar.xml concerning not only the Album but also the Composer e.g. (who was also already written in the file itself, so double the time for tagging), it doesn't work.
For classical listeners like me, it's important to have the same usability for MKV-files like with FLAC-files. Therefore it would be great to have under options the possibility to deactivate the main category "Video" and instead the listing of my MKV-files for each title like the FLAC-files in the same category "Music\Album (Music\Artists, Music\Conductors and so on)", since they are music-files (only with additional motion pictures) and often I hear them without switching the TV on. Maybe I have to explain, that I've ripped the dvd's by title. For e.g. a concert of a classical pianist with several composers: The played sonata is one piece, therefore one file (prior a "mini-dvd/bd" because of the chapters, with MKVs it's possible to have only one MKV-file including the chapters), an etude from Chopin is another title, therefore another file (mpeg/mkv). With the MKVs, it's not longer necessary to use the Sidecar.xml, even the cover can be included like in FLAC-files - JRiver only has to support this correct (compatibilty with the tagging of FLAC-files, in this case with the abovementioned MP3Tag)!