There are 85 leveled books in total which were carefully designed to teach a child to read. Having one voice for the narrational was intentional when we created Ooka Island. The female voice (Auntie Kay) is the role of a teacher who is always asking questions as a child reads and pointing more subtle pieces of the story to pull comprehension.
The man's voice (Zobot) is the narrator that reads the text with the child from the very beginning as a child may not be able to read at this earliest level. To have different voices for the different characters distracts from reading the text and interacting with the teacher's voice. At this stage of early reading, more than 2 voices become more like a movie with a cast of characters.