The story is the same for an STB, a satellite tuner or a cable decoder:
The sound level output of my STB was very low, so when I switched to a normal TV broadcast I had to turn the volume up rather high.
When I then switched to Net TV the sound was too loud!
So, you do not need to decrease the sound volume of Net TV, instead you need to increase the sound level of the set top box.
Simply access the settings menu of the set top box and increase the volume to max.
Now all the sound levels for Net TV, Normal TV, DVD, and other sources are set to the same level.