"Prica jednog dana" is not shown in its real order while playing in Djenka's theater - it cuts from the opening credit sequence directly to the song sequence which is at the very end of the film.
The 'tone film' playing in Djenka's theater is "Prica jednog dana", which was released in 1941, seven years after the events in "Maratonci trce pocasni krug" take place.
In a scene which is showing Hedy Lamarr in Ecstasy, Djenka speaks of Hedy's wealth and success in Hollywood, yet the movie is set in 1935 and Hedy didn't move to USA until 1937 and was unable to reach the level of stardom he was talking about.
The movie is set in the 1930s, but the 'tone film' that Djenka plays in his theater ("Prica jednog dana") was made in 1941. Tone films weren't made in Serbia until 1940s.