Hi Bill,

One idea: Have an audio voltage controlled oscillator, locked to the
audio input with a phase locked loop. I'd try the 4046 chip. Add the
locked VCO output shifted by 180 degrees to the audio. Set the
amplitude of the counter-tone, so that it doesn't completely cancel the
output, but just attenuates it. The PLL will keep the shift constant.
You'll also have to detect when the PLL is not locked and remove the
counter-tone on this condition. No idea how well this would work... but
I'd cost only a few $, for the 4046, an opamp or two and some Rs&Cs.

Anyway, just an idea.
Cheers,
VK3FLP