Thanks, but this is a known thing that's been ongoing for many months. This is one thread about it:
Something must've changed with the Roku device at some point because I never had this problem before. Changing the various audio settings did not help.
Anyway, I just ended up getting an Apple TV, so it won't matter to me anymore, but hopefully you guys can sort this out. Earlier today, the sound got very loud, and distorted my speakers (when using the Roku). Not good.