I've seen certain Japanese-speaking students make their Ds like Ð; even more mysterious (in that asking why lent litle clarification). Very confusing to me personally, if not other teachers, in that Ð is a different phoneme in Bosnian
In Icelandic, thorn is the voiceless sound in 'thorn', 'thin', while eth is the voiced sound in 'this', 'bother'. In Old English, however, they were merely alternatives for the same letter, at the scribe's whim. Whichever was used, it was pronounced voiceless initially and finally, but voiced between vowels.