I have had this problem on a couple titles. We were watching "The Duchess of Duke Street", (A BBC series from the 70s), and to hear the dialog clearly we had to turn the TV way up. There was a considerable amount of hiss when we did this. When the closing music came on, we had to quickly turn it down. It's not that the sound level for the music is too high, just that the sound level for dialog is too low.
Most titles the sound is just fine. I'm connected via HDMI, and I'm pretty sure that it isn't the Roku box. I don't know if it is Netflix itself, or the source material that they are using. The audio of some older DVDs was not mixed properly.