Want device to fall back to 2.0 audio.
Any Roku device picks highest quality sound, based on a user's configuration.
Roku Ultra (Cooper) is the only device that natively supports 5.1. The rest of Roku devices can pass on to TV/sound bar, audio receiver, etc to deliver a higher quality sound.
Wouldn't recommend creating a channel that only offers 5.1 content because there is the chance a user's device would not be able to encode it (which means no audio for the user).