Firstly, you'll need to open up Oculus on your computer

Go to the left side bar and select "Devices" 

Select Rift or Rift S depending on what headset you are using. 

A window will appear at the bottom right of your screen. There will be an option to Hear VR Audio from Computer. Click the slider so that it turns blue to indicate that the feature is now on
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Now navigate to your computer speaker settings by clicking the volume icon in the bottom right of your screen. Select the speaker system that you'd like the sound to be mirrored to

Close and reopen the Oculus application to save your settings

*Note: audio will continue to play through your headset speakers whilst also mirroring to your chosen speaker system.*