Hello Ian,
DDJ-RB engineers saw the issue and answered below.
Please click "MIXER/MIC" in the rekordbox GUI to show MIXER and MIC panels and see if the "MIC ON" button is ticked or not.
When you tick the "MIC ON" button, the letters in the button turns blue and the level meter at the right side of the button will be enabled and MIC input will be available.
Thanks,
CRM, Pioneer DJ