Friend Logs Off
Right click the link to the sample below, choose "Save target as ..." and save it with your other sounds.
This booing crowd will express their disapproval whenever a friend logs off. In order for this to work you must also have the game set up to inform you of such things. Enter the following commands to set up some friends and get the event information displayed. Set up the sound as normal.
You can use the second command repeatedly with different names to add all of your friends. They will also be listed separately when you do a qwho command to list the players currently online.