The way they are now, Apple computers will not let you record your computer audio with any of their built in programs. Most programs that will let you record system audio can be pretty expensive. This is a problem for most people that need a cheap way to record system audio for whatever reason, such as screen recordings, or legally recording uncopyrighted audio. Fortunately, there is a way around this problem.
In this tutorial I will show you an ABSOLUTELY FREE way to record the system audio from your computer (not microphone) using Quicktime and a free program called Soundflower. This is useful for screen recordings that you need to capture your computer's audio with, or legally recording uncopyrighted audio. So let's take a look...
(I'm not as familiar with WIndows computers, but I believe there is another program called "Jack" that should do pretty much the same thing. But I'm not 100% sure.)
The first thing you should do is open System Preferences, and go to Sound. Click on Input. You should see something like "Internal Microphone" and "Line-in". Now click on output. It should say something like "Internal Speakers" and maybe "Headphones" or something. And in the "Input" part. The input specifies where you want the computer to receive audio from when you do some sort of audio recording. The output specifies where you want your computer to play the audio through, or how you want to hear it.
If you notice, there's nothing in the input about receiving the computer audio, and nothing in the output about outputting the computer audio. But don't worry, we can fix that.