If you have a Dell AS501 speaker bar that buzzes after the computer is turned off, you need a new plug-in power supply.  The switching electronics inside the factory-supplied unit are not properly filtered, if at all.

 

But, what you don't need is another power supply of the same kind. The problem is probably a design defect and therefore exists in the entire production run. At least the replacement speakers I received exhibited the very same problem.

 

What you need to do is go to Radio Shack and get one of their plug-in transformers that provides 12v DC @ 1amp.  Splice the output of that transformer into the speaker's power wire (center pin is positive, polarity does matter) and you will have no more buzz.