Hello Roland38,

It's quite easy: I assume that you have a reasonably modern TV with several HDMI connections. Ideally, one of the HDMI connections should be labelled "eARC" or ARC. Please plug the HDMI cable supplied with the Sonos Soundbar into this port (and of course into your Sonos Soundbar). All other devices, for example the Swisscom Box, or your PlayStation or XBox, etc., are connected normally and directly to the other HDMI ports of your TV.

This is why this TV connection technology (ARC) is called Audio Return Channel, because the TV knows that it will automatically pass the audio signal of all connected devices through to the eARC/ARC port, and so your Sonos ARC Soundbar will play back the sound. Works perfectly.

Greetings

I assume that the swisscom box is also connected to the TV? Then everything runs via the TV, but it needs at least one ARC-HDMI connection (ARC = Audio Return Channel) for 5.1, or even better an eARC for Dolby Atmos - otherwise you connect the Arc via an optical cable, but then you have no surround sound.
Everything that runs via the TV then comes out of the Arc, including the swisscom box.