Select the Apple icon in the top left corner of the screen.
Select System Preferences from the menu.
On the System Preferences window, select the Accessibility icon.
On the Accessibility screen, select Audio in the left column.
In the right column, tick the checkbox for Play stereo audio as mono.
When you are finished, click the Close (red) button in the top left corner of the System Preferences window.
Note: If this does not work it could be because someone else manages your computer settings (an IT department or administrator). If this is the case, you will need to contact them to access these settings or for further help.