Only audio is being sent through the HDMI cable to the Arc. The Sonos-supplied cable is sufficient for lossless audio formats like Dolby TrueHD and multichannel PCM. An “8K” cable isn’t necessary.

What would be the sense of bundling a cable with the soundbar that would not be up to the task of transporting the information needed by the soundbar? 

Any HDMI cable with HEAC support can handle eARC. Both the HDMI cable which comes with Beam and Sonos Arc have HEAC support.

Technically, there are four kinds of cable:

• Regular HDMI
• High-Speed HDMI
• High-Speed HDMI with ethernet 
• Ultra-High-Speed

The cable that comes with the Beam/Arc is a High Speed one with ethernet, where the ethernet pins are used for eArc. Quite often referred to as HDMI 1.4+

Personally speaking, I would just use the cable supplied.

Thank you!!!