I don't know how the Roku handles audio tracks BUT is it possible to switch from Surround to Stereo internally if the appropriate track doesn't exist ? I have a .mkv movie with AC3. It the Roku is set to Stereo, no sound and visa versa, if it is set to Surround and there is a stereo track, no sound.
Maybe give some error back to the app or user. Then let him switch like you do with Closed Captions, On/Off. This would be more beneficial than making them close the app (multiple back clicks), change the audio option and then go back into the app and movie (multiple clicks).
I noticed when I played a Netflix movie, it gave an error about the sound selection. I had the unit in Surround, maybe Netflix only does Stereo. But the only hardware option is "Has 5.1" true or false. How did he know ?
I know you can select a track if you have multiple audio tracks and know the track ID. But this requires that you know something about the structure of the video. But even if you knew that the video was Stereo or Surround, there's no way to switch it.