When recording using the internal microphone on a MacBook Pro running OS 10.14.4, the recording levels rise and fall in 4-second intervals. Upon playback the problem is evident. When exporting to audio, the problem remains on playback. It sounds as if a limiter may be in effect, softening levels after clipping, and then raising again to appropriate levels. It is a constant up and down of volume levels.
The built-in microphone on the Mac laptops have an option to automatically process the sound.
Try turning that off.
Audacity has no natural ability to affect incoming sound. It records whatever is provided, distorted or not.
It does have a timer and volume trigger, but that’s it.
Over on the Dark Side, what else do you have running? Some chat, game, and conference systems try to set the volume automatically and not ask you.
Pull up the Dock and see how many things and Apps have little lights or marks under them. The fewer, the better.