According to a few sources based on Old English sounds, Æ wouldve been pronounced like the sound in cat or fan (source: https://people.umass.edu/sharris/in/gram/GrammarBook/Pronunciation.html) and since Ð (or DH in some cases to separate it from TH as in THought and THyme) is basically the TH sound in THis and THough or oTHer (source: https://www.youtube.com/watch?v=tu1t3Fn5Lw8) I'd assume in Old English this name was probably pronounced something like AH-DHEL-RAHD. However, Modern English definitely says it more as EH-THEL-REHD and it's spelt as Aethelred or Ethelred in most modern spellings.
Love how one of my favourite Anglosaxon names ended up being the Name of the Day for my birthday (July 2nd) :).As stated above, this is among my favourite Anglosaxon names and I'm pretty sad some of these names disappeared because of the Normans.
I had a student in my 2nd grade class named this. I was unsure of how to pronounce the name when I got my class list, so I asked his teacher last year. I saw complications with the name all school year. He had long hair, too, which made visitors to our class even more unsure of who he was because the name doesn't help identify gender for most. His family pronounced it "eth-el-red" which I think makes most people who hear the name think "Ethel" and think he's a girl. Because of his unusual name he was picked on a lot too and it was a constant concern I had to watch out for. He proudly told me he is supposed to be related to this historical figure. That may be a far reach, but I do appreciate parents who pick different names. Common is boring. My Aethelred was always an interesting guy!
'Ethelred the Unready is a figure of fun in English history. It is now considered old-fashioned to classify monarchs as good kings or bad kings, but by almost any measure Ethelred was a bad one. In 978 he inherited the rich and respected kingdom of Engla-lond that had been pulled together by Aethelflaed, Edward, Athelstan and the other descendants of his great-great-grandfather Alfred. By 1016 Ethelred had lost it all, from Northumbria down to Cornwall, in the course of a reign that made him a byword for folly, low cunning and incompetence. 'Perhaps the one sphere in which he deserves some sympathy is his unfortunate nickname, a mistranslation of the gibe made after his death by chroniclers who dubbed him Ethelred 'Unred'. In fact, unred was an Old English word that meant "ill-advised", and it made a rather clever pun on the meaning of Ethelred's name, "of noble counsel", rendering Ethelred Unred "the well-advised, ill-advised".' -Robert Lacey
― Anonymous User 6/17/2011
7
"Ethelred the Unready" sounds hilarious, but in fact conceals an Anglo-Saxon pun: he wasn't "unready" in the sense of "always late" or "poorly prepared", but "un-advised" or "poorly advised". This gave him the name + title of Aethelraed Unraed, Wise Advice Bad Advice.
(Source: https://simple.m.wikipedia.org/wiki/Athelred_of_Wessex)