I’m not familiar, but isn’t the Xfinity an infrared ‘remote’? If so the Play:5 speakers will not work, as they do not have an infrared receiver - you would need a Sonos Home Theatre product instead.

Check your TV’s settings, you will need to be able to set the TV’s Line-Out to variable. On some sets you have a second output that is variable instead of a setting switch.

A second option if the above aren’t working is to switch and use the headphone output, that is normally a variable output controlled by the TV remote.

Just be aware that any and all Sonos analog line ins are subject to a minimum of a 75 Ms delay. This is why Sonos never sold the PLAY:5 / Five series of speakers as being home theater speakers.