Be very careful when you plug in a external monitor and a external speaker.
Make sure that all the power cords are in the same strip.  If you don't you
take a chance of having a very bad audio buzz.  This is caused be a "ground loop" and the only way of getting rid of this loud buzz is to make sure that you 
have a common ground.                                
Make sure that all the power cords are going in to the same strip or off the 
same outlet.  This will assure you of a common ground.