Hello sdstarnes welcome to the forum.
Are you getting any sound from the cable box or just no 5.1? Many broadcasts are only in stereo so you must use Dolby PLx to create a surround effect from the stereo source. If the programing is coming in Dolby Digital then your receiver should decode it and give descrete 5.1 surround sound.
Does this model decode Dolby Digital? You said your DVD player is giving you 5.1. Is it decoding the format or are you havin the receiver decoding? I'll look for a manual shortly to see what your receiver will do.