This is a known problem that was caused by Roku’s ‘upgrade’ to OS14. Roku has been little help. You can find the latest on this problem here:
If you want to follow the whole discussion from when it started look here:
There is a workaround:
In the settings for the Roku device, go to Audio → Digital Output Format → Custom, then make sure Dolby Digital is selected for the Dolby section. Can leave DTS off. Then go back to the Tablo app and try again.
For some users this solves the problem, for others, they get picture but no sound. It seems it has to do with the combination of Roku and specific TV.