I guess this tip is Gentoo specific. By default KDE uses FreeDB for getting CD info when ripping CDs. If you want to use MusicBrainz native service (not via their FreeDB proxy), there are several steps you’ll need to take.
First, you should compile kde-base/libkcddb with the musicbrainz use-flag turned on. Next you should go to System Settings->Advanced->CDDB Retrieval and check the “Enable MusicBrainz lookup” box. From my experience MusicBrainz is not a complete replacement for FreeDB as some CDs aren’t recognized by it (but recognized by FreeDB).