I am not sure. I have been using iTunes and it auto populates the composer field when it downloads the info. With media center I have to manually enter each composer which is very time consuming. I have the column set up but all there is is a right facing arrow with a circle around it and no information. How do I set it to auto populate this column?
You should do a tag dump and find out if itunes is writing to the file or not ... it should be. But check (select file and in tag window click on where it says flac fo files that itunes has "tagged") if it is not there in a composer field itunes just kept the info in its library. If it is in the tag and you import the files into JRiver, it should be there.
However if you are talking about getting tag data from JRiver ... or lots of programs for that matter , its absolutely outrageous at the number of tag sets floating on databsaes and the web that do not have the composer fields filled in at all -- unacceptable, but it seems like a lot of people just don't care about composers anymore .. so if its not there it might never have been there is what I'm saying. Then you have to manually enter it (I'd use the tag window for this or a pane in a pane view not the fields in a column) which does stink.
whatever is originally tagging your audio whether you are using, itunes, jriver dbpoweramp, music brainz picard whatever ... make sure it is writing to the files for the fields you want -- especially if they are fields that are not likely to be included in the file name itself. That way if you have to use another player the information goes along with the files.
Edit: Did that make sense? .. I pay extra for my metadata to so called pro databases and I find it outrageous that composer is often just not there .. so I'm sort of ranting here, sorry