I suspect your audio problem had more to do with the size of the neutral than the fact that it was shared. With all of the harmonics you see in electronic power supplies I suspect the neutral was carrying more load than the phases at any given instant. In the example you gave it was clear the neutral was undersized, even if you were just serving light bulbs since you have a potential of 40a of neutral current. Super neutral cable (recomended for electronic loads)would have a #8 for 3 balanced #12 phases.
