Final answer:
The sound technique violated in the movie scene where a distant car backfire is heard too loudly is diegetic sound consistency, which pertains to maintaining a realistic volume level in relation to the distance of the sound source.
Step-by-step explanation:
In the hypothetical scenario, where a car backfiring several blocks away sounds much louder than it should based on the distance to the character, the sound technique being violated is called diegetic sound consistency. Diegetic sound refers to the sounds that logically belong in the film's world and are presumed to be heard by the characters. Consistency is broken when the volume and distance relationship does not match the audience's expectation of sound behavior in a realistic space. This technique is crucial for maintaining the film's believability and immersion. For example, loud sounds from a nearby car should be louder compared to one several blocks away.