Not sure why you want to get rid of the IR link, but Carl's diagram looks like it should work, he's taking the native serial data directly to the sound board, the same way they'd be cabled if you use the ERR cables.
Carl, clever use of limited tether pins by muxing them, nice job! I'm confused as to where the chuff is generated on your diagram, all I see are inputs. Is that square block the chuff switch? Hard to read the diagram with the limited resolution. Why is the chuff switch in the tender, wouldn't it be better in the locomotive on the driver for synchronized chuffs?