Ina ddition to what others have said, a LOT of times, the audio side wiring is at fault. Without getting too far in to it, you should make sure that your shields on all of your MIC wiring are connected only to the shield in the cable. The shell of the connectors should not be attatched to Pin 1 (Shield). A metal jack in a metal, grounded box can produce the dreaded "ground loops". You have to check the mic cables and the mics themselves too! Also, on line level runs from the mixer outputs to amps and equipment, only one side of the cable shield gets connected, snip the other end (I like to lift it at the amps).
There is a lot more to this (a good book is "audio system design" by Phillip Giddings, a good read for any electrician), but in my experience, almost all these problems can be attributed to audio cable problems. If you exhaust all other options, put all the sound gear on an isolating transformer, 120/240, it doesn't have that much load and it does not need 3 phase.
Also, I'm sure you checked this, but a N-->G bond at a subpanel or dimmer rack will raise absolute hell on everything. Maybe your problem is as simple as that.
If you need more info about this just ask, i did sound systems and touring audio for 3 years, and we ran into all sorts of things
Cheers,
Mike
