Geomamma, DS7 used to do this a lot and still does. It used to irk me so much too. And like you, I sent him to the doctor (ear specialist) who pronounced him normal. When asked specifically by the doctor, he said that when he asks a question, he's also busy thinking up what the answer could be, and the voice in his own head is very loud (not a real voice, but a "loud thought"). He can hear me speak but it doesn't register and it stays in the background, especially when he's following a reasoning. He sometimes does a double take and repeats out loud what I say to him verbatim, as if to digest the information I've given him.
If it helps, it does happen less often as he got older, but it still happens frequently enough for his teachers to comment.
