Got myself a Roku 3 for this reason but the whole interface is “ugly” - prefer cleaner Plex interface on the Android platform.
It might not be an easy implementation, seeing how the roku system is smart enough to toggle audio output from TV to the remote.
I don’t even mind if audio output streams on both mobile and my android TV box and i manually mute my TV volume but of course it would be perfect if there’s an additional “night mode” while casting from mobile to TV to automatically switch audio output to the mobile device instead of the TV.