http://www.siliconera.com/2013/08/06...ondercon-2013/
Hideo Baba mentions Ayu in the Tales of interview. It's around 22:20.
Basically he says that they really wanted Ayu to write and sing the song for the series. He also mentions that they requested her to change the lyrics a couple of times and that the recording studio told them that no one has made so many requests to Ayu before like them, but he seems to be very pleased with what they got. No wonder she was back for Xillia 2.