As long as they don’t claim the voice is the original actor (misspell the name perhaps, or the Hollywood classic ‘based on’), they won’t be impersonating no?
The Ford ad didn't say it was Midler, they just implied it by using her song with a soundalike. There was another similar case with a parody ruled as impersonation. I don't think there's good precedent for exactly where that line is drawn.
