In S04E10 of Game of Thrones as Arya is sailing to Bravos, the show's theme begins to play, only it is accompanied by a choir singing a song. This was quite unusual as we never heard that theme music with actual lyrics throughout the entire show.
So first of all, are these actual words sung or is this just vocal melodization? In the former case, what are those lyrics and how do they relate to the story? Are they based on the books as some of the songs from the TV-show are or were they created for the show itself? Even if they weren't actual words, it would be interesting to know why the choice to provide a vocal accompaniment of the show's theme, something we never had before, was taken for this episode.