Either Iceland or Greenland would be where what they speak now would be closest to how the Norse sounded back then. Relatively isolated areas preserve the language at the time they were founded while in cities pronunciation changes the fastest.
Greenlandic Norse died out 5-600 years ago and left no descendants.
The closest living language to ON is Icelandic and Faroese is also pretty close but not so much as Icelandic, likely because the Faroese were forced to write only Danish for 300 years.
That being said Icelandic and Old Norse sounded dramatically different. Almost all of the long vowels changed (á, ó, au, æ, y, ý, and more), all the nasal vowels disappeared (no living descendant preserved them), and the consonant system also changed to a comparable extent.
Wikipedia has a nice overview. This is in addition to the much more minor but still substantial changes in grammar and vocabulary. The orthography is remarkably similar though.
In fact it can be argued that some West Norwegian dialects sound a lot more like ON than Icelandic does even if the grammar, orthography and vocabulary have changed to the point of almost no comprehension of ON texts unlike Icelanders which can read most of the texts with only a glossary.
Haraldr's voice acting is an Icelander just reading ON lines with modern Icelandic pronunciation with some minor changes to make it sound more archaic which is completely different from ON. The way he changes it is inconsistent and way too minor to be considered anything but modern Icelandic.
