AppleTV App No Sound

I have an Xfinity Wireless Pace Xi5 box, a Sony Bravia TV, and a Bose Soundbar. The TV and the Soundbar are eARC compatible. If I understand correctly, the Pace Xi5 is not eARC compatible.

When I try to play an AppleTV show from the AppleTV App (not an AppleTV Device) on either an iPhone or a MacBook I see the video but get no sound. All the software on the iPhone and MacBook is up to date. If I disconnect the Soundbar, the show plays through the normal TV speakers just fine. I have double-checked and triple-checked the connections between the devices - Pace Xi5 to Sony TV, and Sony eARC to Bose Soundbar eARC. I have tried with and without the Bose Soundbar's optical cable.

Are there some configuration steps that I am missing, or do I need an eARC compatible XFinity Wireless box? If so which one?