Need a few more details, please, but if you’re using a Sonos soundbar, you likely need a TV set to create the ARC signal that the soundbar is looking for. Alternately, if you’re using optical, you need to be sure the signal being sent is either stereo, or Dolby Digital, and not anything else, like Atmos, which can’t be carried by an optical cable. 

Hmmm, it’s just a basic sonos one speaker. It seems to only connect to Itunes and the Sonos app.

While streaming Xfinity via the computer, the sound only comes out of the computer speakers and I’m hoping it would connect to the Sonos speaker.

Perhaps, it’s not possible?

How are you connecting to this Sonos One?

If the Sonos is working with iTunes and the Sonos app, that suggests that xFinity hasn’t updated their app with the latest API to support AirPlay 2 streaming. 

One further note. If you have a Sonos PLAY:1 instead of a Sonos One, it doesn’t support AirPlay 2 at all. 

If you are trying to use the One as a PC speaker, read this:

I tried all this and it only works with Itunes. I’ve tried to stream Youtube or Xfinity and it won’t connect to the speaker. Thanks

You “tried all this”?  I’m confused.  That link describes how to use a “Sonos Playbar, Playbase, Beam, Arc, Amp, Move or Roam” as a PC speaker.  You have stated you only have a Sonos One.  I’m not sure how you could have tried all that was in the link when the link specifically states it is only possible with the Sonos Playbar, Playbase, Beam, Arc, Amp, Move or Roam.