Does the cable box have optical out and is it your only source of content?

If so you could use that optical and the adapter supplied with the Beam. 

Thank you for responding. We also want the Beam to work with Xbox so plugging optical into cable box doesn’t work. To use the Xbox we have to unplug cable box and plug in Xbox. 

You need an HDMI switcher with optical output. you need that for the Xbox + cable box anyway, right?

Or buy an optical or hdmi switch. Probably an HDMI switch / audio extractor with optical out would do it.