Are you sure about the cable box requiring an HDMI-ARC connection? Can you kindly provide a link to the online setup/user manual for your particular device/model please.
 

Usually it’s just a straight forward HDMI connection for such devices, or is the Comcast TV box also a ‘receiver’ for attaching surround sound speakers & sub to it aswell?
 

A link to its online manual might help.

An HDMI-ARC port is intended to be used between a display and a sound system. HDMI ports normally send audio and video in one direction from the source to the display, switch box, or a Home Theater receiver. Modern TV’s include “Apps” and one needs a way to send audio from the TV App back to the sound system. This is the function of the TV and sound system HDMI-ARC ports.The TV and sound system each have an HDMI port which is marked as “ARC” and this connection should be used at both ends of the HDMI cable between TV and sound system.

You should connect the cable box to any HDMI input except the ARC port and connect the TV’s ARC port to BEAM.

There may be an option on the TV that needs to be configured before the ARC port is enabled. Depending on the TV model, you will need to enable “CEC”. Unfortunately, this is sometimes given a proprietary name, such as “Anynet” by the TV manufacturer.

As others have said, this is highly unlikely. More likely is that the comcast box requires HDMI-CEC, not HDMI-ARC, and that should be available on all ports. ARC is usually only on a single port.

I have an Xfinity X1 DVR.  I have the HDMI output of the DVR going to the HDMI input of the TV.  I also have the Optical Output of the DVR going to the Optical/HDMI adapter which plugs into the Beam.  I did this combination to eliminate a audio lag that was due to the HDMI-ARC from the TV, which I previously had used, before the Optical.  I have had this configuration working since I got the Beam in December.  My only minor issue with this arrangement, is that I need to push the remote button to turn off the TV and the mute button to no longer hear the volume.   Xfinity disabled the ability to turnoff the DVR from the remote, after having lots of on/off sync problems from customers.

UPDATE:  I later added a JTech Optical switch, so that I could route the sound of the TV apps to the Beam as well.

Check the audio output format of the cable box.

I just purchased two SONOS SL speakers and the Arc. I have the ARC connected to the Xfinity X1 box with the HDMI/Optical adapter cable. Is this the correct setup? How would use the HDMI-ARC without the optical adapter.

I just purchased two SONOS SL speakers and the Arc. I have the ARC connected to the Xfinity X1 box with the HDMI/Optical adapter cable. Is this the correct setup? How would use the HDMI-ARC without the optical adapter.

The Arc is meant to be connected to the TV by HDMI-ARC or HDMI-eARC for Atmos to work. Connecting it via optical will cost you Atmos. Why didn’t you connect the Arc to the HDMI-ARC port via the supplied HDMI cable? 

Thanks for the info. I just upgraded to the LG LG OLED65CXPUA 65” TV. That was not an option with my old Samsung TV.