The Gray Phantom by Herman Landon audiobook.
A woman is apparently murdered in a New York auditorium under very suspicious circumstances one evening during a performance. Helen Hardwick happened to be in attendance that evening, as she had written the play that was being performed, and she was the only person to have caught a glimpse of something peculiar just before the murder. She also heard an ominous laughter which would haunt her for a long time. Was the Gray Phantom involved, or could he assist with the investigation that Helen was to become involved with?