Connecting cables: Xfinity box to Vizio soundbar to tv

You don't mention any model numbers, but it's likely you need to connect devices to the TV and use the one HDMI connection to feed audio from the TV to the soundbar using ARC (Audio Return Channel).

Thank you for reaching out @user_kdh6ew. We normally do not assist with setting up audio devices but the link: https://www.youtube.com/watch?v=LVU8pWHdEnM which is our YouTube channel may help. I would recommend reviewing the manual on your soundbar for additional assistance in setting up the equipment.