The last question is:
I think the first part of the answer to this one is given at the beginning:What can you give to the Hymn of One, and what can the Hymn of One give to you?
The second part of the answer is in the same place:Completing this audition will help us determine the strength of Your Voice.
So the answer to the last question should probably be:They Hymn of One is adding members to the Eternal Choir.
I can give the Hymn of One the strength of My Voice. The Hymn of One can give me membership in the Eternal Choir.