I hear a woman sing really clearly 1:17 into it in the parts "can't see your fance, can't hold you close".
I'm gonna go to sleep and zone out with my ear phones and listen if I can hear anything more.
I don't think that girl is Brandy though - the girl singing in Heaven Can Wait have more of a "cute" voice.
Another reason why I don't think it's Brandy is because this track was produced by Teddy Riley's camp.
Unbreakable was produced by Rodney Jerkins, who is really tight with Brandy - or at least was.
EDIT: Here is the filtered version of Heaven Can Wait, you can clearly hear the harmonies the girl does - she sounds nothing like Brandy
You can hear her clearly 1:58 into the song - I wonder who this "Que" girl is, that is credited.
You can clearly hear Dr. Freeze's background vocals in the beginning of the song also.
http://www.mediafire.com/?2nw5ctnidmg