This is not a TV specific issue and no changing of settings or reinstalling fixes the problem.
It's happening because the latest build of the Hulu app is not enforcing consistent audio decoding between the content and the ads or respecting the decoding format set in the settings.
Hulu is partly to blame for not being consistent with the audio codecs used by their ads (they also have tons of issues with volume leveling and excessively loud ads) but this is something the Roku app should be able to handle.
However, this no sound issue is 100% a problem that started with the latest build of the app and something Roku needs to address internally.