I have had a similar weird experience with sound on my Shield Pro. I have sound on all Streaming Apps (Amazon Prime, Netflix and Disney+) and sound watching OTA on Channels. The only Steaming App with no sound is Philo. This has happened 3 times in the last month.
No sound when watching directly from Philo or when using TV Everywhere from Philo on Channels.
I have two other TVs in the house, streaming from FireSticks and I have sound watching Philo on those devices when the Shield loses Philo sound so I can’t blame Philo.
The only way to get sound back when this happens is to reset the Shield Pro. I tried unplugging and replugging power to the Shield but this doesn’t help.
You need to go the Shield settings (top right corner “Gear” Icon, scroll down to Device Preference then select “About”. Then scroll down to “Restart”.
You might want to try this sequence. I never had this issue before so I’m thinking a recent Philo or Shield update might be the cause.