Keejus, I have confirmation from Michael Curran himself that the Viking DLC music is based on two tunes:
1.
Drømde mig en drøm i nat
"The earliest piece of Scandinavian music thus discovered comes from the Codex Runicus, a vellum manuscript dating from c. 1300 and containing early Danish law texts, most importantly the so-called Skånske lov, or Scanian law. The Codex Runicus is also the source for the melody "Drømte mig en drøm i nat." This melody, thought to be the first two lines of a ballad or folksong, is found on the last leaf of the manuscript, written in the same hand as the text on the preceding eight leaves, but otherwise with no obvious connexion to it. It is the oldest preserved piece of music known in Denmark."
2.
“Nobilis humilis”
"An earlier piece of music which may shed light on Viking Age musical traditions is a twelfth century gymel or "tune in two parts," Nobilis humilis, written in praise of St Magnus in Latin by the monks of St. Magnus Cathedral, Kirkwall, Orkney. St. Magnus was martyred on the island of Egilsay on 16 April, 1117 during a dispute with his cousin, Earl Hakon, over the just division of the Earldom of Orkney. Nobilis humilis is preserved in the same 13th-century manuscript as Ex te lux oritur, the hymn for the 1281 wedding of Princess Margaret of Scotland and Eric II of Norway.
The hymn is in a two-voice polyphony in the Lydian mode and harmonized in parallel thirds. This can also be described as a parallel organum in thirds, one of the earliest polyphonic forms."
Curran found both tunes on a website called The Viking Lady:
http://www.vikinganswerlady.com/music.shtml
Love how they're authentic, not made up stuff like what was done for the Vikings in Civ IV!