Static in sound system

[jdavia]

Most female receptacles today are ground fault protected. That is they have one wide and one narrow slot. The pins on the male plug matches them. No liscensed electrician will wire them wrong. The same goes for the manufactures of the equipment that plugs into them. Therefore I wouldn't first blame the cause for the hum there.
The hum comes from the proximity of the units to eachother, and possibly from the metal cases touching another case, or a bad cable. The fact that when you are on battery and have no hum tells you that the source of the hum is the AC current. Not a necessarily a ground loop. It could be in the multiple receptable you are using. Are these with a ground?
Try pluging the laptop or the projector into another line to see if the hum goes away.