Per the latest article posted on RPGFan, this statement was made without any further detail/explanation:
"Unfortunately, it looks as though Emi Evans, who provided lyrics and vocals in the original game, will not be returning for NieR: Automata."
Soundtrack hype just went down like 80%. I still think Keiichi Okabe and the MoNACA Sound Team can do fine with any vocalist, or with no vocalist at all. But the soundscape they made for NieR was ... I've never heard anything like it. It was okay they didn't try to do it again on Drakengard 3. But for another NieR game? Her voice should at least have *some* presence IMO.