Wow... Aika IS hurting.. ouch.. And either Aika can't sing live, or she's just not trying to sing....
Ok so Aika doesn't get the yukkuri na nagare boshi or whatever line that was. That is all Koharu! Judging from the mic recording....
EDIT: Judging from some of the TV Spots, The "Egao Yes Nuudo" parts go to Yossi, Miki, and Koharu. Then Micchi, Sayumi, Kame, Reina, and Koharu do the Ahs during the chorus parts. I think.... Because I'm thinking Reina might be singing during the Ai/Risa part and Koharu might be singing during the Yossi/Miki part... And the rest do the ahs...