Hi welcome to the forum.
The problem is down to the way your TV handles Dolby Digital audio which the HD channels use.
Try the following
Settings – Picture and Sound – Digital Audio out
Change the setting from Multi-channel to stereo
Turn down the volume of your TV
Turn up the box volume using the remote to maximum
Turn up the TV volume to a normal listening level
Use the TV volume to adjust the sound.
If you do a factory reset remember to restore the box volume to max.