I cannot increase the media volume beyond about 15-20% in a latest install of ATV 9 on my VIM3. I have a Topping E50 connected via USB and I can hear some low volume. However, when I try and increase the sound volume in the settings it keeps defaulting back to about 15%. I can increase the volume for the alarm to 100%. After some googling, some pointed to the Do Not Disturb option on for android devices (but no mention of ATV).
Solved. I found the Khadas IR remote control and I was able to increase the volume to 100%.