I know these products sound stupid, but Ethernet can insert a DC bias and repeat AC-coupled noise from other sources. This includes its own PSU. There are industrial Ethernet products that specifically address both these problems, because in a factory environment, both phenomenon can have practical consequences when there's motors and power electronics throwing around big magnetic fields. I imagine being near speakers with big drivers can also cause some of the same problems that would never be found in a typical desktop or datacenter computing environment.