Hmm, really good questions. Mirri's singing (it's not really singing) is a vocal expression of excitement, wonder, involvement or any other strong feeling. In this story it is a special gift, inexplicable but a basic tenet.The underground living. - I'm quite remiss with this. I worked it out in my mind ages ago when I structured the world settings for Attunga. I should have expounded about it in the first chapter. I'll work it in somewhere along the way.
I planned on a three weekly posting schedule but so far have managed a bit faster than that.