This setting is configured before making the recording. To get to get the menu to enable this click onto the down arrow next to audio on. Please note the Audio will have to be turned on to configure this setting.
Once you have opened the menu options, ensure that "Record System Audio" is selected. This is the option that controls whether the computers audio is recorded.
Please note that this will record all audio from your pc. e.g. notifications of emails, alerts etc.
You are now ready to begin your recording and capture your computers audio within the recording.