I don't remember seeing a switch but I saw something marked SOUND which looked more like an adjustment. I'll have another look tonight.
Do you have a picture of that adapter? Just to add, we don't use coax over here, it would be rf to rf, one side has the circle within a circle, the other has male bit with the prongs on the outside. http://en.wikipedia.org/wiki/TV_aerial_plug#Belling-Lee