I believe that you should be able to hear overtones (2* or 4* mains freq) coming from lightbulbs and power supplies (computers, chargers, etc) in an average room - it could be tested by running a decent mic + solidstate recorder on batteries and checking how loud it is.
That may be true, but there are plenty of things in your home that serve that purpose. Right after reading the first sentence of the article, I started hearing it. Either 60hz or 120hz -- a low B natural. Maybe it's coming from the heating ducts, or maybe it's a fluorescent light bulb. Heck, maybe it's the fridge.
It's really bad whenever I plug speakers into the wall. Not sure if it's the mains, or the radio tower that's 500 feet away, which also "allows" me to listen to NPR at conversational volume without use of a radio.
You can't hear mains hum without a speaker (or transformer acting as a speaker).