Reprint from Nico.
This was posted in one of the iDOLM@STER threads on /a/. It was... really fun to watch, actually. Not exactly one of those-peaceful-to-listen-to vids like the voice actresses' introduction interviews for the first game, but nice nonetheless.
I think it's... Miki's voice actress's birthday? They're... torturing her ;_;
And Haruka's voice actress is annoying.