The widespread use of sound in cinema in the early 1930s naturally led the main industrial players in amateur cinema to extend the innovation to smaller formats. After extensive research, which began in 1933, the Pathé company launched 9.5mm sound prints along with the Pathé-Vox projector in March 1937. The new projector enabled the showing of 9.5mm reduction prints, with optical sound printed on the left edge of film.

The photographic reduction of optical tracks from 35mm posed major technical challenges for the two Pathé engineers entrusted with this task: Léopold Lobel and Georges Boulot. In order to respect the original aspect ratio of standard format images, the optical track from the source 35mm film was reduced in the same proportions as the image. With a reduction factor of 2.5, the optical track would measure just 1mm across in 9.5mm. This meant that the image size on the 9.5mm print would only be slightly reduced. However, such a narrow optical track raised a number of reproduction issues, including the necessity for the finest possible emulsion granulation, in order to best capture microscopic-scale light variations. There was also the difference in running speed between 35mm and 9.5mm (at the same recording rate of 24 frames per second) to be considered, which made re-recording even more complex – especially for high-pitched sounds of frequencies above 4,000 Hz.

It was the addition of sound on 17.5mm Pathé-Rural films that paved the way for 9.5mm sound. The problems were solved in 1932 thanks to Henri Chrétien's patents on anamorphosis, which were employed on the 17.5mm format to compress the length of sound tracks. In February 1934, the magazine Ciné-amateur published, exclusively and with photos, the successful optical transfer of a 35mm sound negative onto 9.5mm. This was followed by three years of research to design the Pathé-Vox projector and an industrial method for manufacturing sound copies.

Until 1957, 35mm sound films were regularly released in 9.5mm versions, mainly in French and English. Film historians Patrick Moules, Garth Pedler and David Wyatt have documented over 500 titles that were made available for the non-theatrical market. Some silent films were reissued in sound, with the addition of musical accompaniment. Sound films adapted to Pathé-Baby, in silent or sound versions, continued to be reedited and shortened, but to a lesser extent than silent films.

The sound projectors produced by Pathé and other manufacturers could project both sound and silent films. To achieve this, they were equipped with a dual frame rate, 16 and 24 fps, and projection masking that hid the soundtrack if necessary. Because of their high price, these machines were mainly intended for schools, universities, film clubs and wealthy consumers.