There have been a number of similar posts. The TV control commands are sent via infrared (IR). The remote's IR sending on some Voice remotes has started to fail intermittently. This can be confirmed by pointing the remote at a digital camera, one in a cell phone will do, and observing the camera screen while pushing the tv control buttons (power, volume, mute) on the remote. If IR is being sent you should see flashes on the camera screen.
See if the solution in this post helps. (https://community.roku.com/t5/Remotes/Roku-Voice-Remote-TV-Controls-Stop-Working-Potential-Fix/m-p/8...)
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