static discharge from my finger caught radio signal ?

The thermostat you touched may be protected from ESD (electrostatic discharge) by a capacitor (instead of a TVS or other semiconductor). The capacitor would have been charged by the ESD, and discharged more slowly through protection diodes within the integrated circuits inside the thermostat.

When the protection diodes were biased 'on' they would also act as RF detectors. Other circuitry inside the thermosat may have helped to amplify the strongest AM station (received at your house), and some device inside the thermostat acted as a minature speaker.

This explanation is true (based on my knowledge and experience with similar phenomenon), and as correct as I can think, without actually investigating the thermostat. Further explanation would require me to see the circuit and layout for the thermostat.

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My opinion/guess is that the radio is probably microprocessor controlled. When you snapped the arc, the resulting transient disrupted the radio's proper operation. I've seen many modern devices turn themselves "on" due to such a transient. One second later, the firmware which controls the radio (but is not the actual receiver) loops back to the top and discovers the radio isn't supposed to be on because the "power switch" is really off. It then powers down the receiver. This is what comes of the modern approach, where programmable devices can control their power. They're never really "off" because the power is never completely interrupted. The "on/off" button is simply an input to the "smart" circuitry which ultimately controls a power triac, or some such device. My HP all-in-one C3180 printer-fax-scanner works like this and turns itself on or off at random, when the thermostat in a nearby small space heater cycles. Now I leave its plug pulled except when I want to actually print something.

There was a big flap over this very thing because a particular brand of automatic coffee maker would mysteriously turn itself "on" while empty and supposedly "off." Homes burned down and people died. Nobody did a damned thing about it.

Furthermore, this approach cumulatively wastes an enormous amount of power which would be saved if "off" really meant OFF.

Any radio frequency noise from a static discharge should sound like, well, static.

If you are hearing voices, then either: (a) a surge of voltage from the power company just happened to jump the gap across the power switch in your radio, briefly turning it on; (b) your radio was briefly powered for a moment from a strong radio transmitter (it could happen, if someone with a CB radio was passing close by your house at that moment); (c) you are hearing sound from somewhere else echoing off of your speaker; or, (d) you are imaging things.

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Hey, I like the answer from "thewhiz" below. I didn't think of that one, since I am from the old school of analog radios.

Hi!

Some excelent answers here already. The one thing no-one addressed though, was how can you actually HEAR the radio though - No loudspeaker.

My own suspicion is that the spark itself was able to produce the sound. Not many people realise that sparks can be used as loudspeakers. They are actually supposed to make very good ones, since there is not the inertial of the cone to "color" the tone of the output.

Interesting observation you made!

It is not radio signal