Roku ECP Volume Control of Attached Device via CEC

Volume can only be controlled on a TV with Roku built in.  You can not control volume on a connected Roku STB.

https://developer.roku.com/docs/developer-program/debugging/external-control-api.md

Some Roku devices, such as Roku TVs, also support:

  VolumeDown
  VolumeMute
  VolumeUp
  PowerOff
Roku TV devices also support changing the channel when watching the TV tuner input:

  ChannelUp
  ChannelDown