To record internal computer audio, you need to install plugin Record It Audio Device on your Mac.
There are two setting options to record your internal computer sound. One option is Auto setting, and the other is manual setting.
Auto Setting (Recommended):
1. After successful installation of Record It Audio Device, 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.
(Note: this option also needs the successful installation of Record It Audio Device)
1. Go System Preferences -> Sound.
2. Click the Input -> Internal Microphone.
3. Clike the Output -> Record It Aggregate Device.
4. Open Record It, select an area, then choose Record It Audio Device in audio sources.