I've been listening to this a bunch of times already.
It's feels so beautiful and profound in both the ears and soul.
Árstíðir is a band from Iceland. Here they are singing the 13th century Icelandic hymn “Heyr himna smiður” at a train station in Wuppertal, Germany.
More about the band here: http://www.arstidir.com, they don't usually sings hymns.