What is Gunshot recording?

A gunshot recording is used by Andreas to alter the timeline of the murder of Olesen. Andreas kills Olesen with a silenced pistol, then plays a tape cassette recording of a gunshot which sounds hours later. Patricia inducts that the timing of the murder must be off since the killer could not be seen entering or exiting the room. As a result, based on her reading, she suspects a recorded gunshot is what was used. Upon examination, K2 does indeed discover a gunshot recording.