To record internal computer audio, you need to install plugin Record It Audio Device on your Mac.
If your macOS is older than 10.12, please install Record It Audio Device here.
There are two setting options to record your internal computer sound. One option is Auto setting, and the other is manual setting.
1. After successful installation of RecordItAudioDevice, you just need to open Record It;
2. Click Screen on the homepage and choose the area you need to capture;
3.Then in Audio Sources option, tick System Sound under Audio Sources(Auto), then Record It will do the rest for you automatically;
4. At last you can click Recording Button to start recording.
Manual Setting(Not Recommended):
(Note: this option also needs the successful installation of RecordItAudioDevice)
1. Go System Preferences -> Sound.
2. Click the Input -> Internal Microphone.
3. Clike the Output -> Aggregate Device.
4. Open Record It, select an area, then choose RecordItAudioDevice in audio sources.