If you want to record the internal audio of OBS, Add a New Source and select “Audio Output Capture”. Select the default device, or whichever output device you are using. 

If you can see the audio meters moving, but can’t hear a sound, you might need to use a monitoring system, or a Multi-Output Audio Device. For Mac, you’ll need to download Blackhole, go to the apple Audio MIDI Setup app, then create a new Multi-Output device that contains blackhole and your main speaker output. 

Select this device in the Audio Output Capture source in OBS. 

Check you are receiving audio signals in the metering of the audio mixer. Make sure the audio meters aren’t going into the red or your signal will be distorted and sound low quality. 

Now when you record, any sounds that would be coming out of your speakers will be recorded into your video, this is a great way of recording from video games, conference calls, browsers, applications and more.