AirPlay should be supported on up-to-date devices: Use Apple AirPlay software feature to watch Xfinity Stream on compatible smart TVs or devices - Xfinity Support
If they support just AirPlay 2, but not AirPlay 1, then the audio without video is one of the things that might happen. They may have changed their support for receiving devices, or something else may prohibit AirPlay.
To what model Apple TV box are you trying to stream?
Identify your Apple TV model - Apple Support
Find your Apple TV model number - Apple Support
Apple TV HD (4th generation) or later supports AirPlay 2. Apple TV (3rd generation) or earlier may have different compatibility per content provider support.
Else contact Xfinity Support for assistance with this feature, and any conditions or procedures that may apply.