In NDI Webcam, click on the gear for your input and under the Audio menu there are some options. I'd try setting Channels 1,2 and changing from Automatic Levels to a specific level (of course not mute). See if any of those changes help.

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.