I am running the audio through a Vizio Sound Bar via an optical cable. It ain't the Vizio Sound Bar, as I get the same audio problem if I run the audio through the TV speakers. The feed from the Roku to the TV is HDMI.
I went through the manual - some models have audio settings that only pertain to HDMI but not this one. It's strange that this occurs with both the TV and the sound bar (and I'm assuming only with the Roku) - I suspect it is a digital audio issue. The best way to confirm that is to connect the Roku directly to the TV using composite (R/W/Y) and playing some non-HD titles.
The manual does warn not to use both the 'optical audio out' and the 'composite audio out' at the same time - so, make sure you're not doing that. When you 'run the audio through the TV speakers' do you physically disconnect the audio cable from the TV to the sound bar (I'm guesising you're using the optical out) ?? Disconnect it if you're not, to test with the Roku.
There are potentially more factors with feeding the audio from the Roku to the sound bar (depending on how receiver-like the sound bar is) - which is why I'm trying to isolate the issue with the Roku connected directly to the TV. If the issue persists with a plain RCA composite connection it's likely something at fault in the Roku (despite being your second one!).