The Game of Thrones music wiki describes the music thus: I have heard elsewhere that there is some recognisable Spanish in there, but I don't speak Spanish, so I can't confirm this. Without anything further, I think we have to assume there are no lyrics available, as there are likely no actual words.
He was asked a similar question in a reddit thread, and only stated that the choral voices were 'Valyrian-influenced', but gave nothing further. The music is called 'The Children' and, like most (if not all) of the GoT music, it was composed by Ramin Djawadi.
