A 2020 iMac running Big Sur 11.2.1 and Audacity 2.4.2
It was working fine but now isn’t. iMac system prefs Sound does get input so it isn’t a physical connection problem. Sound prefs are set to USB AUDIO CODEC for Input and Output (which used to work). Tried changing Output to iMac Speakers (or headphones) and that didn’t help. Audacity settings don’t have many choices. Audio Host is Core Audio (the only choice). Recording Device is USB AUDIO CODEC (only other choice is iMac Microphone). Playback Device is either USB AUDIO CODEC (which used to work) or iMac Speakers (which doesn’t work either). Software Playthrough is On. Click to Start Monitoring is clicked. But regardless of all that, when I start Record there is nothing… just a flat line. There may have been an OS update since I last successfully used Audacity.
After I uninstalled Audacity and reinstalled it the problem was solved. While the settings that used to work still don’t, at least changing to other available settings does work now after the new install.