and the DMX was running down channel 42.
Not totally on point, but as long as all your snake lines are properly balanced and shielded, you shouldn't get any crosstalk from the dmx signals. Since DMX is basically a constant digital RS-485 signal at its roots, the active dimming of lights wouldn't change the signal being sent through the snake anyway. I too have done this before with no problem.
The typical problem with hum in a sound system from lighting dimmers tends to come more from the actual dimmers themselves - typically the construction of the dimmers will determine this, though anything that isn't properly wired/grounded could still cause backfeed into the sound system if they're on the same power source.