How accurate does Hollywood portray how people historically sounded and spoke, especially in the medieval period?

It's pretty obvious that the dialogue depicted in the show Vikings was completely incorrect since it's not an Icelandic show, thus while the Vikings were speaking their native Norse, we hear it in English. The script was also written in modern English as if people in the modern day were speaking in an old English dialect. A lot of the characters had a southern American accent(floki) or a northern English accent(Ivar) or even an Irish accent(Bjorn). The only character who sounded like a real Viking would have sounded like was Floki whenever he would yell out in anger.<br><br>I've seen this same method done in a bunch of other period dramas or historical epics, for example a documentary on the Normans starring Stephen Fry, which was made by the BBC with an all-British cast, so there wasn't an accent issue of a non-British person playing a historical person from the British Isles. But the fact it was a BBC show meant that there were a bunch of British people with their modern British accents playing people from the Norman epoch like William the Conqueror. <br><br>It also seems like people in period dramas often have a completely accurate knowledge of who their ancestors were and what they did. In Vikings, Ragnar Lothbrok knew where he was from(farmer from Kattegat), who his father was, and all about the history of the Vikings and their Gods. In reality, people didn't know that much about their ancestors, and Vikings had oral traditions which were often lost over time. So it doesn't seem like people actually behaved like that in the past.