Decrease your box and tv volume to zero.
Now, tune in to HD channel & turn on DD 5.1 Audio.
Now, increase your TV volume up to the level you want to keep or is OK for you on HD channel with DD 5.1 ON.
Now, change channel to SD channel and increase box volume up to the level you want to keep or even 100% if required.
Here after increase or decrease volume of TV when ever needs.
You can control box volume with DishTV remote only when you access HD or SD channels in normal PCM ( Stereo ) audio mode
To control volume on HD channels with DD 5.1 mode Universal remote is recommended.
Universal remote let's you control your TV volume when you access HD channel in DD 5.1 mode.