Can anyone point me in the direction of a gauge that plays a sound when crossing outer marker, middle marker, inner marker? Searched with no joy.

:-hmmm Don

You are correct but I was looking for an invisible gauge to put in a panel that would enable the sounds on by default.

It's already there, it's not invisible though, click the little avionics square on any default panel and it will open the avionics panel and than just click marker beacon audio.