In Rogue One: A Star Wars Story, Disney/Lucasfilm had chosen to CGI the character of Grand Moff Tarkin. However the voice acting done for Tarkin sounded a lot like the original actor Peter Cushing. Did they just find someone else that sounded similar to Peter Cushing? Or was this special sound editing?
Actually, the filmmakers hired Guy Henry (he portrayed Minister of Magic Pius Thicknesse in Harry Potter and the Deathly Hallows) who has a superficial resemblance to Peter Cushing. He played the role of Tarkin and the VFX team then changed him into Cushing. In regards to his voice, the VFX sound team then probably added a simulation of Cushing’s voice with all the new dialogues.
According to the video below, a "source close to the film" confirmed that Guy Henry did provide the voice for Tarkin as well as the visual. I don't know how reliable that is and it also doesn't give any information about whether there was any post production sound editing done to make his voice sound any more like Peter Cushing's. Its not unreasonable to assume it was nothing more than some good quality voice acting, but I personally would like to know more details as I was wondering this same thing.
The particular section that mentions this starts at about the 2:50 mark
The voice for Governor Tarkin was provided separately by Stephen Stanton. See this article.