I have the same issue. Roku Express linked to a basic Samsung TV. Set up a few weeks ago. Initially volume is fine, then may start to decrease to just above a whisper even when the TV volume is at 100%. There is no volume control on the remote. This happens regardless of show or streaming service or audio settings on Roku or TV. Volume is fine when we switch the TV back to antenna or DVD player. It seems to reset back to normal when we go off Roku for a minute or more, but as we return to watching it creeps in again. It is a pain and we are thinking of returning the Roku Express.
I just purchased a new Roku Express+ today for a 22 inch Sharp TV we have in our Kitchen, I am noting the same weird behaviour. I have an older Roku Express in my bedroom and it has the complete opposite issue where the Volume is Very Loud. I watch YOUTUBE TV on both systems and they are complete opposites. I wish they would play "normal". If I put my kitchen TV on the highest volume with the ROKU it still is not loud. I updated the firmware in the new unit today as well as the channel. Any advice appreciated. Thank You