SOLVED! Have experimented with my setup (outlined above) and discovered that the “fault” is not with NDI. The problem with no audio was solved when in OBS I changed Desktop 2 output from Disabled to Default. So NDI seems to be fed from this. Not sure if this will be the same in all cases or whether in some setups output 1 will be used as the path to NDI (as you would expect) - but maybe this will help other users with a similar problem.