When the police found the body of Professor John Smith, they found a tape recorder & gun by his side. On starting the tape they heard a suicide message, closely followed by the sound of a pistol being fired. Without hesitation the police handed over the case to the murder squad . Why?
I presume the tape recorder had been switched off, which cannot have been accomplished by the Professor, and must therefore have been carried out by his assassin.
The recorder must have been rewound, for the police to know straight away. If the tape had stopped before the finish, the could have been a power failure (mains or battery). The Prof. must have been shot or else the tape would be as useful as a sound-effects tape. If the tape was left to run, the sound of a person exiting would have been heard.