The Wiki says:
Vorbis/Ogg frameworks are used by some optional plugins and are not shipped with either OS X or GemRB distributions, but there are native Mac versions available.
Could someone specify what I need to get so that GemRB can use ogg-files on OS X?
And can GemRB use ogg files in all places where the game plays a sound/music? And if so do I need change the resource references in the gamefiles to point the ogg files?