I haven't seen the movie yet, but I finished the show multiple times and have never seen either Ikaros' or Nymph's ears. Why is that?
The protruding robotic wings on Nymph's head and Astrea's head are their ears because they have been used to communicate. Following this principle whereby the bell-shaped objects on Ikaros's head receive speech, those wings are their ears. I have never seen actual ears but those seem to fill the job. Also it shows that Ikaros uses those as ears which I hope provides some of the answer you are looking for.