Is you sound static-y? Screechy? Always whining? Here are a few things to check...

1) Make sure all audio cables are plugged in all the way.  Even if they look like they are, push them in further to see if they snap.  Another thing to try is moving the audio cable from the back of the computer to the front or vice versa.

2) Audio interference can and often is electromagnetic disruptions. If the Promethean board audio is the issue, I'd make sure that the audio cable isn't running parallel to any power cords.  Audio cables are best ran perpendicular to power.  It could also be the cable itself is damaged and the ground wire(s) that protect from the interference are no longer properly connecting.  The reason you may only hear this interference when the computer is plugged is plugging in the cable allows the electromagnetic energy to complete a circuit. 

3) The audio cable itself could be bad. Try getting a replacement aux cable and running it from the computer to the speaker/board. Don't worry about running it through the ceiling... just plug it in and see if it works. If this takes care of the issue, use a new aux cable. 

Sadly, if none of the above fixes help, this problem is most likely originating from the speaker's themselves.  Any one connection that is underground in the whole assembly can cause static.  This can often happen with cheaper speakers.