Hi @Styrken
Welcome to the Sonos Community!
Does the Beam start sounding like the TV speakers, or is it actually the TV speakers that are playing instead of the Beam? I’ll assume the latter.
I see no reason why this would only happen with a particular Apple TV app, though I’m unfamiliar with Viaplay - I certainly recommend checking any settings available in the app that might relate to power, sound or other CEC options (Consumer Electronics Control is what allows Home-Theatre related devices to send commands such as change volume and power on).
Failing that, I recommend you unplug your TV from power for at least 2 minutes. This will restart it’s software in a way that standby does not. It’s probably worth rebooting your Apple TV at the same time.
Finally, I recommend you get in touch with our technical support team, who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.
I hope this helps.
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein