What's a word for a voice doppelganger?
Solution 1:
soundalike (or sound-alike)
a person or thing that sounds like another, especially a better known or more famous prototype:
a whole spate of Elvis Presley soundalikes.
Also, vocal doppelganger is used colloquially. There is even a few results in Google books.
Solution 2:
I'd call it gesangsdoppel, which is German for vocal double and sounds kind of neat.
Solution 3:
voice double (n.)
Like stunt doubles or digital retouching, voice matching — also known as "voice double" or "soundalike" — is a tool the movie business uses to conjure the fantasy we imagine in our heads. It's a process in which voiceover artists are hired in postproduction to come in and double for the voice of a star. "Here's the trick directors use when they can't get stars to finish a movie"; businessinsider.com
Before the release of The Unholy Three, many stories circulated stating that Chaney wouldn't speak in any role or that he might use a voice double (although the technique for dubbing had yet to be perfected). Michael Blake; A Thousand Faces
The audiovisual contract permes audiences of mainstream films to expect that a character will sound very different when she sings than when she renders dialogue. Specifically, a character will have a different voice in the song sequence than she does in other, nonmusical sequences. In the 1940s, Bombay filmmakers tried to match the singer's voice to the voice and personality of the actor. This practice of voice-casting quickly faded as playback voices came to be lauded for their own qualities, rather than for being a voice double of the actor. T. Whittaker and S. Wright; Locating the Voice in Film
Nevertheless, when Durâteu starts to act as a "voice double" under the pseudonym Tristan Vox in Felix's radio show, the voice and body form a perfect match in one single person. Mikko Keskinen; Audio Book
Rather than employ a voice double for his Norwegian star, Hughes fired Greta Nissen and replaced her with an unknown Jean Harlow, in the role that made the latter made the actress a star. Arne Lunde; Nordic Exposures