You can solve this pretty easily. First, record the victim, and tamper with the recording as needed. Call this tampered recording "sample A". You need this recording to be somewhere that ambient noise will not vary much throughout the day.
Next, go to the same room (or a similar room) at a time when the victim will not have a good alibi and when the room will have the same basic ambient noise. Record using the same recording device in the same conditions (e.g. if it was in your pocket for the original recording, keep it in your pocket for the second recording). This is "sample B".
Take your recordings into Audacity. On sample A, apply the "Equalization" effect like this: http://i.imgur.com/gxPTV.png
Now mix the two samples. As long as the ambient noise does not contain easily isolated components like other people's voices, you'll have a convincing forgery.
Now you might argue that there could be other forensics techniques to detect this kind of tampering, but I would argue that if such reliable alternative techniques were available, this mains analysis technique wouldn't be particularly valuable in the first place.
I don't think that's taking into account the harmonic frequencies. The 50Hz buzz is not a sine wave. That means that even with that filter, both 50 Hz mains sounds should still be detectable in the recording. Unless I'm missing something?
Next, go to the same room (or a similar room) at a time when the victim will not have a good alibi and when the room will have the same basic ambient noise. Record using the same recording device in the same conditions (e.g. if it was in your pocket for the original recording, keep it in your pocket for the second recording). This is "sample B".
Take your recordings into Audacity. On sample A, apply the "Equalization" effect like this: http://i.imgur.com/gxPTV.png
On sample B, apply the opposite filter like this: http://i.imgur.com/6qeYQ.png
Now mix the two samples. As long as the ambient noise does not contain easily isolated components like other people's voices, you'll have a convincing forgery.
Now you might argue that there could be other forensics techniques to detect this kind of tampering, but I would argue that if such reliable alternative techniques were available, this mains analysis technique wouldn't be particularly valuable in the first place.