I had a quick look at my G4 last night, and I found that if the settings/TV signal quality/HDMI audio source was set to surround the audio adjustment via the AD/vol control was not available, changing the setting to stereo it became available.
This is probably the reason why some people are not able to adjust the volume, is this a fault or a design feature?
I have just been connected, the sound on my youview box is very quiet i need to have the volume on 25 for comfortable hearing, is this normal for this box, when switching back to tv i have to make sure the sound is lowered.
It sounds like the volume on your YouView box has been set to low. If the Remote has been programmed to operate the TV it may have first been used to adjust the volume on the YouView box, down. Increasing this should solve the problem.
As TimCurtis shared in post 6, by pressing and holding the ‘AD’ button on the remote control and pressing the Vol+ Button you can raise the volume of your YouView box.