Thing is, like Bill says, we are "just MC 12 users".
JRiver did not add native flac support until build 12.0.331 and therefore Taxee cannot expect to receive any official support unless he/she is running the latest available build, which is currently build 365.
1. Under Tools/Options/File Associations, see if the Flac checkbox is checked. If not, check it.
agreed. I assumed this was done already.
2. Under Tools/Import Folder, see if the Flac checkbox is checked. I'm not sure whether this setting is library specific or global to all of MC 12, so be sure it is checked for the library you are using. If not, check it.
The import folder settings are folder specific. You can choose which file types MC will look for in each folder. The thumbnails, fix broken links etc. etc. options you see under the folder list are library maintenance tools and affect the entire library.
3. Are the Flac files you can't play listed in the file list for the audio view scheme? if not, are they listed under some other file type? There have been discussions of problems where Flac files were imported but not recognized as audio files.
This could be the most likely root of the problem. They won't be in the library as another
file type, but it's very likely that they will be imported as a different
media type, namely documents, rather than audio. If this is the case, right click the tag action window and find the [Media Type] field in the "Also Show" list. Click on it to have it added to the tag window. Select all flac files imported as documents then click on the Media Type field in the tag window and change documents to audio.
4. Note the location and file name of a Flac file you can't play. In Windows Explorer, right-click on that file and choose "Open with". Select Media center 12 from the list. if it isn't listed, browse to the Program Files folder when MC 12 was installed and choose MC12.exe.
MC12.exe is not present in the program directory, it gets installed to Windows/System32/MC12.exe
The executable in the program directory is called Media Center 12.exe
If MC12.exe plays the Flac file from Windows Explorer, your problem is probably that you didn't have the Flac checkbox checked (1. or 2. above) when you imported the Flac files. Find and select all those Flac files you can't play and press delete. In the dialog that appears, click on the top radio button "Remove from media Library Library". (You don't want to physically delete the Flac files but just remove them from the MC 12 library. Then re-import those files. (If you want to be extra careful, make a copy of the Flac files before you remove them from the MC 12 library.
In my opinion, these are redundant steps if the files are imported under the incorrect media type, see my response to point #3 above for the simplest remedy.
It's also important when removing/re-importing files that attention is drawn to the "Ignore files previously deleted from library" setting.