Forensic scientists have come up with the answer: they can authenticate these recordings with the help of a hum.
Electric find
For the last seven years, at the Metropolitan Police forensic lab in south London, audio specialists have been continuously recording the sound of mains electricity.
It is an all pervasive hum that we normally cannot hear. But boost it a little, and a metallic and not very pleasant buzz fills the air.
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Rebecca Morelle looks at forensic audio and phonetics on Frontiers on Wednesday 12 December 2100 GMT on Radio 4
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"The power is sent out over the national grid to factories, shops and of course our homes. Normally this frequency, known as the mains frequency, is about 50Hz," explains Dr Alan Cooper, a senior digital forensic practitioner at the Met Police.
Any digital recording made anywhere near an electrical power source, be it plug socket, light or pylon, will pick up this noise and it will be embedded throughout the audio.
This buzz is an annoyance for sound engineers trying to make the highest quality recordings. But for forensic experts, it has turned out to be an invaluable tool in the fight against crime.
While the frequency of the electricity supplied by the national grid is about 50Hz, if you look at it over time, you can see minute fluctuations in the order of a few thousandths of a hertz.
"It's because the supply and demand is unpredictable," says Dr Cooper.
If millions of people suddenly switch on their kettle after watching their favourite soap, the demand for electricity may outstrip the supply, and the frequency will go down.
But when electricity generation is greater than demand, the frequency will rise.
"The grid operators will try and compensate for this, but you can sometimes see some very significant fluctuations," explains Dr Cooper.
Silent witness
A decade ago, a Romanian audio specialist Dr Catalan Grigoras, now director of the National Center for Media Forensics at the University of Colorado, Denver, made a discovery: that the pattern of these random changes in frequency is unique over time.
By itself, this might be an interesting electrical curiosity. But when you take into account that most digital recordings are also embedded with this hum, it becomes a game changer.
Recordings made close to electrical power sources pick up a hum
Comparing the unique pattern of the frequencies on an audio recording with a database that has been logging these changes for 24 hours a day, 365 days a year provides a digital watermark: a date and time stamp on the recording.
