I have 2 Echo Show 15s that have the upgraded firmware to act as a Fire TV. These devices run all the same apps as other FireTV devices, including the Tablo legacy app for my Tablo legacy devices. I have a Gen4 device and the Gen4 app loaded on these Echo Show 15s, but while there is video there is no sound. When I try to adjust the volume I get a short beep and can see the volume bar onscreen but there is still no volume. The sound is fine on the Tablo legacy app and legacy devices.
I have submitted a ticket but just wanted to post here in case any one else has a similar situation, or has found a work around.