At times I had no audio on PBS, or some segments had audio, others didn't. Found the solution. I sometimes watch my local Spanish TV station and switch language to Spanish to have Spanish captions. To solve audio problem I needed to go back to my Spanish channel and change language back to English.
Also note that when you go into the sounds section and select other than the auto setting you might get a message that says some sound may not be heard if the app does not provide the format requested. In my case it as DD + .