How do I get the digital audio input to use the STB digital audio signal?

My HP Pavilion m7250n Media Center computer has a digital audio input
RCA jack on the back. It is right next to the parallel port, not on
the video/audio cards.

When I run an RCA cable from the digital audio output on the
Scientific Atlanta Explorer 3250HD set-top cable box to the digital
audio input on the computer, there is no sound.

This computer has the Realtek HD Audio Manager system.

how do I get the digital audio input on the computer to use the
digital audio signal from the set-top box?

Thanks for any info,

Lew