First of all, I have tried to turn off the surround sound. 
Some recorded shows have no sound when watching on the tv. But those same recorded shows have sound when watching through the Tablo Apple app.
Live shows have sound.
Some older recorded shows have sound, but not newer ones.
Thoughts?
I know that if you turn on/off the surround sound that you won’t be able to hear the previously record. Currently we have 5.1 on and can listen on any device in or home that supports 5.1 (except on our Amazon Kindle 10" tablets) That includes Roku/Amazon Fire stick Lite/ Apple tv/ iPhones/HP laptops.
How is your Apple TV device connected to your HDTV? Directly via an HDMI cable?
What is the make and model of your HDTV? It should support decoding 5.1 AC3 audio.
Sorry, what device are you using when this happens? An Android phone by chance?
It’s not an Apple TV, it’s a Tablo app on my iPad that connects to my Tablo over my network.
Which iPad? What is the model number of the iPad? What iOS version is it running?
The iPad Tablo app should support audio both when surround sound is turned on and turned off.