You can’t fix that with noise reduction. Something is clipping (or otherwise distorting), and it is happening before the audio gets to Audacity. Zoom into the waveform around 2.53 seconds and you can see it.
Describe your recording chain in detail, from microphone to computer.
The headphone socket? That’s an output, you want an input to the mac. What mac? Laptop or desktop? What year and model?
What make and model of “amplifier box”?
Have you checked the batteries on the wireless mic?