SONOVOX, serendipitously appeared in each. The first was the Kay Kyser horror comedy You'll Find Out (1940), the second the Australian SF action-adventure film released almost forty years later, Mad Max (1979). In the latter film, a character who seems to have suffered some sort of trauma to his throat uses the then anachronistic Sonovox as a voice prosthesis, unavoidably lending his speech that hollow metallic ring that makes the sound created by the Sonovox so distinctive, if slightly chilling. (Indeed, the makers of the films featuring "Mad Max" seem compelled to portray a vast array of prosthetic devices throughout the trilogy.)
The Sonovox was an electronic novelty used exclusively to create vocal effects. Hence, it was a device of extremely narrow application (unlike the vocoder), which eventually led to its demise and its interest to us now as a museum piece. A couple of posts back I wrote about Bell Labs' voder, an electronic speech synthesis device. What's unusual about the Sonovox is that it uses two biscuit-sized microphones placed on either side of a person's throat to simulate electronically synthesized speech: thus, it was used to simulate a simulation. An easy way to simulate the artificiality of mechanically created speech, the invention quickly led to anthropomorphized trains, steam shovels, pianos, and vacuum cleaners, that is, to its application in the field of children's entertainment. In yet another serendipity, the voder and the Sonovox were introduced to the general public the same year, 1939 (see my earlier post on the voder for historical information). According to this article from Time (24 July 1939), the Sonovox was invented by Gilbert Wright (the son of the hugely popular early twentieth-century novelist Harold Bell Wright) around January 1939. During a meditative moment while Wright was scratching the whiskers on his Adam's Apple (that is, his laryngeal prominence), he noticed the unusual sounds emanating from his mouth. If you've ever seen Bruce McGill ("D-Day") in National Lampoon's Animal House (1978) play the William Tell "Overture" by drumming his fingers on his windpipe, you get the same general idea.
In this clip, actress Lucille Ball is shown demonstrating the Sonovox in a British Pathé newsreel dated 25 September 1939, although the apparatus isn't referred to as such, the newsreel instead being titled Machine Made Voices! So far as I've been able to discover, the Sonovox made its first appearance in the aforementioned Kay Kyser haunted house comedy You'll Find Out (November 1940). In this film (which also starred Bela Lugosi, Boris Karloff, and Peter Lorre), Lugosi employs the ethereal vocal effect of the Sonovox to simulate the voice of a revenant spirit during a seance, and it is later used by the Kyser Orchestra singer Harry Babbitt during a song, as shown in this clip from the film available on YouTube. The apparatus was perhaps most famously used as the anthropomorphized voice of the train, Casey Junior, in Dumbo (October 1941) but was demonstrated to comedian Robert Benchley in the Disney feature The Reluctant Dragon (June 1941), during a tour of the Disney Studios facility in Burbank. (According to the Time article, Disney wanted to buy the exclusive rights to the Sonovox.) Not surprisingly, the Sonovox was used on several children's records, including a couple issued by Capitol Records in the late 1940s: Rusty in Orchestraville (1946) and Sparky's Magic Piano (1947). It was also used as the voice of the airplane in Whizzer The Talking Airplane (Jackalee Records, 1947). I haven't had the opportunity to confirm the use of the Sonovox in the Forties films The Secret Life of Walter Mitty (1947) and A Letter to Three Wives (1949), but a few on-line sources indicate it was used in these films. No doubt it was used in other films for sound effects. Apparently it was employed well into the 1960s by some radio stations around the country as a way of producing aurally distinctive station IDs, which is to say its application became even more specialized. Given that its application was so narrow, it's no wonder it was eventually displaced by the vocoder, and hence became a historic artifact.