The newest version Audacity in my mac cannot record audio on display (It records my voice through mic very well). It happened to the older version of audacity too. I tried to adjust devices and other settings in the audacity and the mac system settings. It still does not work.
I found an error message saying: the plugin ausoundisolation failed to register, when I tried to enable it. Not sure if this is the reason.
Actually, I should say I cannot record voice through “built-in microphone”, which means the voices like the video on youtube and the people who were talking through skype from the other end. All these voices cannot be recorded with the current Audacity.