This sounds to me like a ground loop problem.
Sometimes when people are grounding their recpticles
they go to a different ground than the AC Panel.
What I ould do first is lift the ground on the
Audio equipment if the cord is using a gorund.
ALso it could be transformer interference. I
ran into this problem when the customer had halogen
lights and the transformer was in the ceiling.
When my wires was up against the transformer
I had a hum. Once I moved it away it was fine.
When running a liine level audio anywhere over
50 feet we always use shielded mic cable. Whenever
it is over 100 feet we use a line level amplifier.
Hope this helps.

Mike