This is an old (11th century) Icelandic song, transliterated into the Roman alphabet as 'Heyr himna smidhur' (if you don't mind me substituting 'dh' for the curved d-thing with a line through its stem) and which translates to 'Hear, Heavenly Creator'. The group is apparently called Arstidhir (again, me doing my best to write the name when my keyboard doesn't have all the right letters).
This is seriously beautiful. Again, it's a reminder that we Catholics have some of the best music - AND that all of the old music was the really good stuff. What we sing now just doesn't move the soul like this does. Even atheist commenters on YouTube cannot help but acknowledge the otherworldly beauty of this song.
Bonus fact: they are singing it in a German train station. You can see a sign saying 'Ausgang' ('Exit') behind them at one point. They lucked out on the acoustics - the echoing of their voices in that area only adds to the haunting quality of the song.
That's another reason why the old styles of churches were better. The acoustics of the building allowed for beautiful music like this...
In Pace Christi,