A few years ago, I was putzing about on the computer when I noticed an odd, soft sound coming from one of the speakers. It sounded like voices, and there weren’t any sounds currently playing on the computer. I take a closer listen, and it was clearly one of the local radio stations playing.
Stephen King mentioned a similar phenomenon in The Tommyknockers, but the idea of glassware and whatnot picking up and playing radio signals doesn’t sound very plausible. Knowing how AM and FM signals are decoded and the original waveform reproduced, though, not even hearing it on my speaker sounds very plausible.
AM signals aren’t terribly difficult to decode. All you need is a rectifier. If the signal is strong enough, sometimes even ordinary metal objects can do the job, as point contact rectifiers sometimes occur accidentally, under the right sorts of conditions.
All you need to decode radio signals is a rectifier and a filter. As QED pointed out, these things occur naturally. If you want to have some fun look into how folks back in WWII made their own radios. Some of them were pretty creative. It goes to show you how you can make a radio out of darn near anything if you are creative enough.
There are two types of computer speakers, those that have the amplifier built in and those that don’t. The ones that have a built in amplifier are much more prone to picking up stray radio signals. Every amplifier needs a power supply, and every power supply contains a rectifier and filter. It’s not that difficult to accidentally make a radio receiver out of a power supply circuit. Worse, any audio noise that gets decoded from the power supply gets amplified by the amplifier circuit (after all, that’s its job, to amplify things) so you end up with an even greater noise level coming out of your speakers.
Any first year EE student is taught to be very careful when using diodes in a circuit, because of their natural tendency to act like an AM receiver.