If you've spent any time around the world's most/least favourite social networking site lately, you'll have noticed that each new notification is accompanied by a cheery sound that jumps you out of your slumber and interrupts your current musical playlist of choice.
To switch these sounds off, click the white cog icon on the toolbar at the top of the screen, select Account Settings and then open up the Notifications page. Click the View link at the top then untick the box labelled Play a sound when each new notification is received. Click Save Changes and enjoy the silence.