How to Make a MacBook Pro Output in Mono

Click on the 'Apple' icon located on the left side of your menu bar.
Select 'System Preferences' and choose 'Accessibility.'
Click the 'Audio' tab under the Hearing category.
Check the box that reads, 'Play stereo audio as mono.' This sends mono audio to both the left and right sides of your speakers and headphones.
Close the Accessibility window and your settings will be automatically saved.
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