Not sure how you can adjust the volume of the box. I have found that some channels seem louder then others. I always have the volume on the Youview box set to 100 and 15 to 20 on the TV in living room but if dads watching I have to turn TV volume up to 25.
In my bedroom I have TV volume set at 25 sometimes 30 depending on what I’m watching as although TV in my bedroom is a good TV sound don’t seem as good as the bigger Samsung TV in living room. I also use a Youview box in bedroom and funny enough I also got the volume of that box set to 100.