It was a method used by movie studios during the early talkie period (1928–1931) to make sound films for foreign markets.
To make an international version, the studio would simply insert (on the soundtrack) music over any dialogue in the film and splice in intertitles (which would be replaced with the appropriate language of the country).
They were meant to cash in on the talkie craze; by 1930 anything with sound did well at the box-office while silent films were largely ignored by the public.
Starting in 1931, studios began to subtitle films, a method still in widespread use today and both "International Sound Versions" and "Foreign Language Versions" quietly disappeared, although the latter method continued to be employed until the mid-1930s for special productions until it became feasible to dub films into foreign languages.
Surviving examples of "International Sound Versions" include "The Shakedown" 1929, "Song O' My Heart" 1930, "Phantom Of The Opera" 1929, "The Lone Star Ranger" 1930, "Men Without Women" 1930, "All Quiet On The Western Front" 1930, Rain or Shine 1930, etc.