So, an audio clip has apparently been leaked of Meryl singing the Witch's "Lament" from the upcoming Into the Woods. It was posted on a Stephen Sondheim blog and you can check it out here.
It sounds very much like Meryl so I'm inclined to assume it's legit. If so, that's likely MacKenzie Mauzy (Rapunzel) speaking at the beginning of the clip. I don't know if it will somehow be removed due to a Disney copyright infringement. Regardless, Meryl's voice, despite the poor quality of the audio, sounded wonderfully light and clear. SO excited for this movie. Into the Woods is set for a Christmas Day release in cinemas.