Note you need to choose a line-in for this, not a four-position port meant only for connecting a headset. In Audacity's Device Toolbar select Built-in Input or similar as Recording Device.Adjust the Input volume slider so that the top 3 bars of the Input level meter do not light up on the loudest passages, but the rest of the bars do light up.Leaving the Sound preferences panel open, start sound audio playing on the computer (this could be from Apple Music/iTunes, or streaming audio, or some other source).Click on the Input tab, then select 'Line Input' from the 'Choose a device for sound input' list.Click on the Output tab, then select 'Line Out' from the 'Choose a device for sound output' list.Choose Apple Menu > System Preferences and select the Sound preferences panel.

As a workaround, you could attach a single to double stereo connector to the output port provide an extra jack for speakers or headphones. At this point you will no longer be able to hear what is playing on the computer.Using a cable with a stereo mini-plug (1/8') at each end, connect the headphone output port to the line input port.This may result in some slight increase in noise and distortion. It has the disadvantage that the audio signal is converted from digital to analog, then from analog to digital, instead of staying in the digital domain. This is a simple and effective method of recording what is playing on the computer.

Mac record screen and audio