This is an Apple issue. To the best of my knowledge, you can not send video to one AirPlay 2 target, and audio to a second AirPlay 2 target.
You could potentially use AirPlay 2 to send the signal to an Apple TV, and use the Apple TV to send the audio via HDMI to your TV, which should then send the audio via ARC to your Sonos Arc.
If there is no audio when sending the AirPlay 2 signal to the Apple TV, you may need to double check the audio output settings on the Apple TV. If the standard Apple App on the Apple TV works properly, then your issue is somewhere in the Apple aairplay 2 world, and not in Sonos.