System sounds (for example, error and email notifications) – along with all other running applications – are automatically sent (as part of the default mix) to the default output device destination, assuming no other sources have been specifically sent to that device. Your default output device is the one with the text ‘Default Device’ under its name in the Destinations panel.Try removing the source or sources playing to the default output device (hover over the source/s in the ‘Now Receiving’ area for the default output device, and click the red x icon) – that device will then revert back to playing the ‘default mix’, which includes your system sounds.The default output device is set in the General Preferences.To mute your system sounds, hover over the system sounds icon in the ‘Now Receiving’ area of the default output device destination, and click the speaker icon. Click the icon again to un-mute system sounds.