Humax boxes analogue audio (and digital pcm ) are volume controlled by the box remote control. Normally you would turn up the volume to near max and use the TV volume control. If you are using a TV for the audio turn off the surround (multi-channel) in the box TV and Audio setup menu. That way HD and SD channels will have the same levels.
I had inadvertently pressed the down button on the volume control on the Humax remote control. I never normally touch this – it must have happened by accident.