To record internal computer audio, you need to install plugin RecordItAudioDevice on your Mac.
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;
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.