"Closed Circuit" (1978 Italy)
From IMDb review:
A group of people arrive to watch a movie. Suddenly when a shot is heard from the movie someone is shot for real inside the cinema, and the police arrives to investigate the murder. Who killed him, and who is the killer?
The police ask everyone to sit in the same positions as before and try to watch the movie again; but, when the gunshot is heard from in the movie for the second time, another person is shot for real! this makes things more complicated for the police as there is no motive or suspects or evidence of gun found!