Mine was exactly the same until the TV engineer pointed out that I had the volume on the freeview remote down low. Just adjust the volume to as high as it will go with the freeview remote and it should be the same volume when you switch over.
Everhelpful, my freeview remote has a volume up and down button but it is a Humax HD recorder with Freeview. A simple Freeview box might require the use of a menu option.