In its original form, eth was pronounced like the th sound in words like this, that or the, or then.
Here is something you never imagined or thought of in the alphabetical world. There used to be some letters in the alphabets that actually went missing or not late included in the final compilation of the English alphabets and they are:
YOGH (Ȝ)
We can thank the Scots for this letter having existed, however briefly. Think of the ch sound in Loch Ness Monster, or the way you'd pronounce the ch in 'challah bread.'
Ethel was similar to Ash in that it was a strange hybrid but was pronounced like the 'oi' in 'join.'
Breaking news: This letter is still used in modern Danish. If you're interested in partying with this antique, head straight to Denmark. Otherwise, just know that it was a short vowel sound in Old English, like the strange love child of a short a and a short e, like when you can't tell if someone said 'pat' or 'pet.'
This was the precursor to today's uu; it lost favor when writers and printing presses started smushing two trendier u letters together.
Thorn was also pronounced with a th sound, but in a softer manner. Imagine using the th sound in the least aggressive way possible, with it rolling smoothly from behind your teeth.