Do you mean you get the sound when you press a button wich is signed in only 1 client ? If so, make a blank button in the WoW client where the button does not have to do anything and bind the button you want to use in your other client to this blank spot. This way the char wich the button is for just does whatever you bound to it and the other should not do any thing .. No sound, no action.

Hope it helps and otherwise this post got me again a few mins closer to the end of the working day!!