it's a good bet that the mysterious voice is possibly a misdirect. Either a soundalike voiceover from someone who is not Stewart, or even Stewart himself, hired for the express purpose of giving some aural clickbait to get people to continue to talk about the film until it happens. We know that there are certain clips seen in films that don't end up in the final movie. It just depends. Raimi made his bones (pun intended) in horror fare. I wonder how much if any he was allowed to tinker with the orthodoxy of the Marvel Studios formula?