Thanks, Hookey.

I think I did try Audacity's preset Noise-Reduction -- at least the levels that popped up automatically -- but when I tried (twice) to apply them to the audio file, I couldn't detect any noise reduction.


I'm not sure why my built-in webcam on my MacBook is getting this sort of hum now. It didn't seem to do that in previous years, but the original one did break down a while back (sometimes recording no audio, sometimes producing noisy static), so I replaced it.   The replacement part may be slightly defective too.
But I think maybe if I set the sound input level for less than the maximum, it might reduce the hum.
Alternatively, maybe if I use an external mic input, that will avoid this problem, but that's not always feasible.   Many of my videos are recorded when the music is unamplified and there may be multiple performers, so a single mic wouldn't work unless it was a condenser mic, and even that won't work too well with a spread-out group of performers.

As for the plug-in line hum filters, do they prevent the hum when the mic is built into the laptop?