I completed Waking Nightmare quest by not killing Erandur, destroyed Vaermina's staff and made him a follower. Was adventuring with him for a while, then I dismissed him. I had some other adventures in the meantime, even met him wandering in the wilderness during that time at which point he already had no dialogues and couldn't be talked to. He would move his mouth as if uttering some line, but he has no voice, not even a generic line. I thought nothing of it at the time, however recently wanted