Re: voice synth, I think that's just an artifact of how birds vocal chords (or whatever gizmo they've got in there) work. The timbre of the voice is going to be different than that of a primate.

EDIT: you can even hear it in their natural calls, they have a buzzing, saw-like texture in the upper-mid and high range.

I think he sounds like a voice synthesizer because the owner probably "trained" the raven to say nevermore by letting a computer program repeat that word occasionally over long periods of time. Thus he learned to mimic the synthesizer sound, not a real human saying it. You can also hear the bird repeat some notification sounds a "waka waka" sound that sounds like Pacman.

I understand that Raven's can vocalize, I am just skeptical of that original video you linked. I am just wondering if there was evidence that it was authentic since that Youtube user's videos seem to go so far beyond what I've seen Raven's do before.