Hi SHUTRcz, Welcome to the Forum!
You need CODECS that allow WMP to recognize the FLAC files since FLAC is not natively supported by WMP.
The lightest and best codec pack is Win7 Codecs
. Also if you have 64bit Windows 7, there is a 64bit tools pack you need to be able to switch to the 32bit WMP so all files will work. Codec writers have been slow to develop 64bit codecs so we need to use the 32bit player that also comes with 64bit Windows.
Anyway, install the Codecs and the x64 tools if needed, only the codec pack if you have 32bit Windows 7. If you have 64bit Windows 7, after you install the tools go to Start - All Programs - Windows 7 Codecs - 64bit tools - default media player, and click the "set 32bit WMP as default" and then the "set 32bit registry item" to make 32bit WMP the default player. There is NO performance difference between the two players but 64bit is NOT supported by many codecs.
For Flac tag support, get WmpTagExtender
and install it. Make sure it's enabled in WMP, go to Options in WMP, plug-in tab, click Background and it should show in there with a checkmark, check it if it's unchecked.
Now you need to re-scan your library so Flac will show, WMP should do this automatically but make sure.
Hope this helps.