I came across this news article today that illustrates one of the other sources of audio and video evidence used in legal proceedings. In this case the source was from recordings made during the interview of a suspect (now defendent) and it was introduced into his trial to help establish his state of mind - in other words, whether he was "legally insane" at the time of the murder.
As you can now see, audio and video evidence is not only obtained from surveillance or recovery (e.g. from the scene of the crime, nearby ATM cameras, etc.). As a matter of fact, in recent years recording of police interviews has become increasingly common throughout the world, so this source of audio and video evidence will also become more common in trials.