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Originally Posted by Don

That's Sarah Alainn singing (or sounds like her, at least) and she recently posted a photo on Facebook (Nov. 13) with Mitsuda about something they were working on but couldn't quite say what it was. Perhaps this is it?

Working on a big project with @yasunorimitsuda Stay tuned!
光田康典さんと新たな作品を制作中☺発表はまだ先ですが、「合唱」を録音〜是非お楽しみに!
今週のラジオのテーマも「合唱」✨ウィーン少年合唱団とコラボしたクリスマスソングも流します! S

Confirmed: http://www.famitsu.com/news/201601/19097510.html