Also solved.   I'm using a 2008 model Mac Pro. I tried restarting and removing the computer power cord then restarting, restarting then removing the USB cables for 15 seconds, plus a bunch of other things. The only one that has actually worked is pulling out the power cable of the monitor itself while everything is running. Plugging it back in has then caused Display Audio to reappear in the list of inputs and outputs.  Might help someone maybe!