If you open that up it will just be an adapter that connects the rca to the cat5 wires and back again, nothing smart about it and definitely won't work through a router (although it will have minimal lag).
but surely you'd also get massive interference with anything on the line?!
I think the general idea with Audio over Ethernet is you run a dedicated line and so it's probably only really useful for running audio signals over large distances*
but surely you'd also get massive interference with anything on the line?!