Of course, and a lot of microphones don't pick up that low anyway. That's immaterial because of harmonics. While the AC wave itself is nearly a perfect sinuso

Of course, and a lot of microphones don't pick up that low anyway. That's immaterial because of harmonics. While the AC wave itself is nearly a perfect sinusoid, various devices introduce all sorts of nonlinear properties, and produce higher frequencies that are harmonics of the base powerline frequency, and thus vary right along with it. These are insidiously hard to remove. Your best bet would be to encode all the voices with a vocoder, ditch the background sounds entirely, and play the coded voices back. Which of course makes it obvious that the audio has been altered... I suspect that similar artifacts will be visible in lights too, since the brightness of an incandescent or neon source varies with the AC wave. Some fluorescents too, though LEDs all have DC drivers so they'd make it more difficult.