We have a winner!
The explanation is correct. I really thought all the references to sound and voice would make it fairly easy, but apparently nobody picked up on them.
(I also specifically wanted it to be 4 lines long as a super-obscure reference to the fact it’s a 4 mana 4/4, but I didn’t actually expect anyone to pick up on that. Just mentioned it as an extra clue in my final hint to see if anyone would notice.)
I’ve always liked this card, and use it in my favourite deck (Vespyr). An echo is a distorted repetition of sound, and the way the Voice of the Wind “echoes” minions by creating a Vespyr minion in response to you playing another minion always struck me as rather poetic, so I wanted to make a riddle about it.
Apparently it was way, way too hard though.
I think my riddle-making career is over. I’ll stick to solving them from now on.