Cause of background buzz in my walkie talkie example?
I have not checked the RF24Audio library but, to the best of my knowledge, basic Arduino boards have no digital-to-analog converter.

The digital-to-analog conversion is probably performed by using a PWM output. Isn’t that background buzz you are hearing the frequency of the PWM output?

EDIT: if my guess above is correct, you may reduce the background buzz with a low-pass filter at the output of the receiver arduino.