Most if not all audio hums are cause by AC current on the grounding between the two points of where the audio equipment is connected into a system, in most cases if a wiring system in installed in a NEC compliant fashion there will be no hum but there are a few cases where this is not always possible and one of them is where the audio equipment is supplied by two or more transformers and the grounded conductor has been grounded at both locations as required by the NEC, this is because the grounding conductor is paralleled with the grounded conductor and some of the current will flow on the grounding, in these cases you have only one or two choices to eliminate this current in a safe manner, one is to install a 1 to 1 low level audio transformer in the audio cable that will break the DC current path of the shield as current takes all paths and any current in the grounding between two points that is paralleled by the audio shield will also have this current on it, the other is as posted in the OP is to install a balanced power supply which the only conductor that is grounded is the center tap which is in the middle of the 120 volt ends which cancels out the current on the shield since both ends of this transformer sees the center tap at 180? out of phase to ground.
Another source of the hum can be when we mix cable TV signals into an audio systems again this is because the cable shield is bonded at both the pole grounding (MGN) and the building grounding which will parallel the service neutral and the grounding the audio shield system can be again paralleled to this current.
Now any current on the grounding when the above is not present can indicated a grounded neutral somewhere in the wiring and should be found as this would be a NEC violation but finding it can be a chore but starting by turning off breakers is a good method to narrowing it down.
