Perhaps it's specific to TV brands. With both my Insignia and Sharp Roku TV, OTA volume is usually around 15-20, while YouTube needs to be turned down to 8-9 and Amazon is usually around 12. Both TVs can get very loud. Since these are both used in a den-like setting, we are sitting no more than 5-6 feet from the TVs. Makes me think it's the audio amplifiers used in the Hisense, not the Roku firmware.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.