If I purchase Roku speakers for a bedroom can it be set up so the sound ONLY comes from the Roku speakers and not the smart tv?
Yes, once your Roku speakers are installed/paired goto the RokuTV settings.
Settings > Audio > Speakers
Select: Roku speakers
Note: With Auto selected initially the TV speakers will have audio output until the Roku speakers connect.
The Roku wireless speakers won't work with any other tv then a Roku tv, you can't use it with any other smart tv. You can now substitute a Roku tv for Roku Smart Soundbar and then connect to non-Roku tv.