Mark Rehorst wrote:

I added a 3 terminal voltage regulator, reducing the audio amp supply
voltage to about 10V. Buzz all but gone. I have to press my ear to
the speaker to hear any buzz now.

The distortion in the audio seems to have gone up a bit, so maybe an
amp that can operate from a lower voltage would be better.

Killing the audio amp is a very effective way of eliminating all sorts
of QRM/QRN. Just kidding, good luck with it.

-Bill