A6/A8 speakers, microphone priority function has no special button. To turn off the function, press and hold the Single Song Loop button for three seconds (the long elliptical button) is to turn off the microphone priority function.
Microphone priority is a function of the public broadcasting preamplifier, that is, when the microphone volume reaches a certain point, the background music is immediately reduced, as to how much to reduce, is controlled by a sensitivity knob.