Answering my own question here. Info from manufacturer. The Marker Audio is ON whenever any of the three ILS/MLS channels are selected. Marker Volume is not adjustable but is pre-set external to the Audio Selector Panel.
So, to hear the Marker Audio you have to - Select any ILS/MLS channel, light on, and turn down the volume to get rid of any ILS Ident that may be there.
This has to be the worst/clumsiest audio design of all time
!