Sonos Amp is designed to connect to your TV via HDMI-ARC, not to your video sources the way traditional receivers would.  Assuming you are playing the Skybox video through a TV, check to see if the TV/projector has a HDMI-ARC port on it.  If you aren’t using a TV/projector, or the TV/projector doesn’t have an HDMI-ARC port,  you will need to connect via optical connection, or you’ll need a device that can convert ‘normal’ HDMI to HDMI-ARC, like an HDFury Arcana.

If you don’t have hdmi-arc or an optical out you can use and just want the sky box audio, I used an Amazon basics hdmi to optical/rca extractor for a while between an Apple TV hdmi out and a monitor hdmi in to get an optical out to connect to a Sonos Amp.

https://www.amazon.co.uk/gp/product/B07KRWYN4R/

Had no lip sync issue with it and it just worked plugged between the Apple TV out and the monitor in.