If the sound seems strange, it's because they had no sound recording equipment with them in Spain. All sounds were added in post-production. Another point of interest: Ernest Hemingway himself reads the voice-over narration. Orson Welles was originally hired to do it, but he and Hemingway had some issues:
Wednesday, August 20, 2014
The Spanish Earth
Fans of Hemingway (or anyone who's seen Hemingway and Gellhorn) will know that Hemingway, along with Gellhorn and Dos Passos, went to Spain to report on the Spanish Civil War, in which the newly established Republic (the monarchy had recently abdicated control of the nation) was under siege by the general Franco, a fascist (I use this word in the technical, not hyperbolic, sense). Franco received aid from Mussolini-led Italy and the Third Reich. Hemingway and Dos Passos, with financial support from others (including authors Dorothy Parker and Lillian Hellman), went to Spain to film a documentary that would bring the plight of the Spanish Republic to the American people. That film is The Spanish Earth.