I just bought a new Roku 3920R, and I do not know how to adjust voice via App. On my iPhone, the app does not show the below buttons for voice adjustment.
Using the official Roku Mobile App on my Motorola phone (Android), I have to use the volume controls on the phone itself to control the connected Roku. (the physical buttons on side of my phone)
It’s a little kludgy but hit the mic button on the app and say mute, volume up, or volume down.
I think Roku (sadly) does not support the buttons directly on the app because it only works in certain situations (type of Roku, type of connection to tv etc)